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•  THE 








3o,  to-  2 


8    BROADWAY,  LUDGATE,  E.C.  4. 

[The  sole  rights  of  translation  in  English  remain  with  Scott,  Green-wood  £?  Sen.\ 




ORIGINALLY  issued  as  a  volume  of  the  series  on 
pigments  and  colouring  matters  by  the  present  author's 
father,  the  necessity  for  a  new  edition  afforded  a  welcome 
opportunity  of  revising  "Earth  Cofours."  Although, 
in  the  nature  of  things,  little  progress  has  been  made 
in  this  subject  itself,  there  was  a  good  deal  to  add  in 
connection  with  the  mechanical  appliances  for  treating 
the  colour  earths  and  manufacturing  them  into  pigments. 
In  other  respects,  too,  the  work  has  been  carefully 
gone  through  and  brought  up  to  date,  with  new  and 
additional  illustrations. 

The  author  desires  to  express  his  thanks  to  the 
various  firms  who  have  afforded  him  assistance  in  his 
task  by  furnishing  illustrations  and  descriptions  of  new 
machinery,  together  with  other  information.  It  is  hoped 
that  this  third  edition  will  meet  the  approval  of  those 
interested  in  the  subject ;  and  the  author  will  be  glad  to 
receive  supplementary  information  to  render  the  work 
more  complete  in  the  event  of  a  future  edition  being 
found  advisable. 





INTRODUCTORY        .......         i 


THE  RAW  MATERIALS  FOR  EARTH  COLOURS      .         .         8 

(A)  White  Raw  Materials  and  Pigmentary  Earths  .        1 1 
Limestone  (Calcite,  Limestone,  Chalk)  .          .        1 1 
Gypsum  (Alabaster)    .          .          .          .          .18 
Barytes,  or  Heavy  Spar         .          .          .  19 
Talc,  Soapstone,  Steatite       .                                      20 
Clay          .          .          .                    .          .          .21 

(B)  Yellow  Earths    .          .          .          .          .          .23 

Brown  Ironstone          .          .          .          .          ,23 

Ochre        .......        25 

Yellow  Earth 26 

Terra  di  Siena    .          .          ...          .          .27 

(C)  The  Red  Earths 27 

Red  Ironstone    .          .          .          .          .          .28 

Bole 31 

Alum  Sludge      .          .        ..          .          .          .32 

Mine  Sludge       ......        32 

(D)  Blue  Earths 33 

Azurite,  or  Ultramarine       . .          .          ,          -33 
Vivianite  .......       33 





(E)  Green  Earth  Pigments  ....       34 

Green  Earth       ......       34 

Malachite  .          .          .          .          .          -35 

(F)  Brown  Earth  Pigments         ....       36 

Umber      .......        36 

Asphaltum          .          .          .          .          .          -37 

(G)  Black  Earth 38 

Black  Schist       ...  38 

Graphite  .......       38 


THE  PREPARATION  OF  THE  COLOUR  EARTHS      .         .       40 
Crushing  Machinery  .          .  .          .          .          .        43 

Crushing  and  Sifting .          .          .          .          .          -77 

Calcining          .          .          .          .          .          .          .81 

Mixing  and  Improving        .          .          .          .          .81 

Moulding         .......       85 


WHITE  EARTH  COLOURS  .         .         .          .         .87 

Caustic  Lime   .......       87 

Pearl  White ,94 

Vienna  White  .......       95 

Chalk 98 

Precipitated  Chalk     .          .          .          .          .          .      107 

Calcareous  Marl        .          .          .          .          .          .no 

Gypsum  .          .          .          .          .          .          .          .in 

Kaolin,  Pipeclay        .          .          .          .          .          .112 

Bary  tes,  or  Heavy  Spar       .          .          .          .          .119 

Carbonate  of  Magnesia       .          .          .          .          .123 

Talc        .          .          .          .          .          .          ,          .124 

Steatite  or  Soapstone  .          .          .          .          .125 




YELLOW  EARTH  COLOURS        .         .         .         .         .127 

The  Ochres 128 

Calcining  (Burning)  Ochre  ....      132 

Ochres  from  Various  Deposits     .          .          .          .136 

Artificial  Ochres         .          .          .          .          .          .138 

Ochres  as  By-products        .          .          .          .          .146 


RED  EARTH  COLOURS     .          .          .          .          .          -151 

Bole 152 

Native  Ferric  Oxide  as  a  Pigment         .          .  154 

Iron  Glance     .......      154 

Hematite          .          .          .          .          ...  155 

Raddle    .          .          .          .          .          .          .          .155 

Burnt  Ferric  Oxide  and  Ochres    .          .          .          .158 

(a)  Burning  in  the  Muffle         .          .          .          .158 

(b)  Caput  Mortuum,  Colcothar         .          .          .160 

(c)  Calcining  Ferric  Oxide       .          .          .          .161 
Ferric  Oxide  Pigments  from  Alum  Sludge      .          .164 


BROWN  EARTH  COLOURS          .         ...         .         .168 

Terra  di  Siena 168 

True  Umber    .          .          .          .          .          .          .170 

Cologne  Earth  (Cologne  Umber)  .          .          .173 

Asphaltum  Brown  (Bitumen)       .          .          .          .174 


GREEN  EARTH  COLOURS           .         .  .  .  .176 

Green  Earth,  or  Celadon  Green  .  .  .176 

Artificial  Green  Earth  (Green  Ochre)  .  .  .180 

Malachite  Green        .  181 




BLUE  EARTH  COLOURS    .          .          .          .          .          .183 

Malachite  Blue  (Lazulite) 183 

Vivianite  or  Blue  Ochre      .          .          .          .          .184 


BLACK  EARTH  COLOURS.          .          .          .          .          .185 

Graphite  .          .          .          .          .          .          .185 

Black  Chalk 194 



COLOURS      .         .         .         .         .         .         -197 

White  Earth  Colours 198 

Yellow  Earth  Colours          .          .          .          .          .200 

Red  Earth  Colours    .          .          .          .          .          .200 

Brown  Earth  Colours          .....      200 

Green  Earth  Colours  .....      201 

Blue  Earth  Colours   ......      202 

Grey  Earth  Colours  .          .          .          .          .          .      202 

Black  Earth  Colours  .          .          .          .          .202 

INDEX  .          .          .  .          .          .          .  ,   203 




BOTH  from  the  chemical  and  practical  standpoint 
it  is  necessary  to  divide  pigments  into  clearly  denned 
groups,  the  following  classification  being  adopted  on 
the  basis  and  natural  history  of  the  substances  con- 
cerned : — 

(i)  Pigments  occurring  native  in  a  finished  con- 
dition, and  only  requiring  mechanical  preparation  to 
fit  them  for  use  as  painters'  colours.  (2)  Pigments 
which  are  not  ready  formed  in  Nature,  but  contain 
some  metallic  compound  as  pigmentary  material, 
which  requires  certain  chemical  treatment  for  its  full 
development.  (3)  Pigments  which,  in  contrast  to 
these  two  groups,  contain  only  organic,  and  no  in- 
organic, constituents.  This  last  class  comprises  all 
the  natural  vegetable  pigments,  together  with  the 
large  group  of  colours  obtained  artificially  from  tar 
products,  fresh  groups  of  which  are  being  continually 
introduced.  Nowadays,  there  is  no  longer  any  strict 
line  of  demarcation  between  the  natural  and  artificial 
organic  colouring  matters,  it  being  possible  to  produce 
even  those  of  the  vegetable  series,  such  as  madder 
and  indigo,  by  artificial  means. 


Whilst  this  group  of  colours  exhibits  the  greatest 
variety,  and  is  constantly  being  enriched  and  increased 
by  the  progress  of  colour  chemistry,  the  case  is  different 
with  the  first  group,  the  natural  earth  pigments.  Here 
we  have  chiefly  to  do  with  the  preparation  of  materials 
occurring  in  Nature,  or  with  bringing  about  certain 
chemical  results,  so  that,  consequently,  the  range  of 
variety  is  far  more  restricted,  and  there  is  little  or  no 
possibility  of  increasing  the  number  of  these  colours 
by  the  manufacture  of  really  new  products.  The 
earth  colours  nevertheless  have  a  high  technical  and 
economic  importance,  on  account  of  their  extremely 
valuable  properties,  coupled,  for  the  most  part,  with 
low  cost. 

If  the  term  "  earth  colours  "  were,  strictly  adhered 
to,  the  present  work  would  have  to  be  confined  to  a 
description  of  the  physical  and  chemical  properties 
of  the  various  pigments,  and  of  the  various  means  by 
which  they  can  be  brought  into  suitable  condition 
for  use  in  paints. 

However,  of  late,  the  term  has  found  wider  applica- 
tion than  formerly,  since  it  has  been  found  practicable 
to  modify  (shade)  certain  of  the  earth  colours  by  simple 
operations,  and  thus  considerably  increase  the  range 
of  tones  of  the  substances  known  as  earth  colours. 
The  progress  of  chemical  industry  has  also  largely 
increased  the  number  of  the  so-called  earth  colours, 
certain  methods  of  chemical  treatment  having  enabled 
substances  that  are  of  little  use  for  other  purposes, 
to  be  employed,  in  large  quantities,  as  pigments.  The 
application  of  these — usually  cheap — by-products  is 
still  further  facilitated  by  the  fact  that  they  can  be 
transformed,  by  a  simple  chemical  treatment,  into 
pigments  which  are  distinguished  by  their  beauty  of 


colour  and  at  the  same  time  possess  the  great  advantages 
of  durability  and  cheapness. 

As  an  example  of  this,  mention  may  be  made  of 
iron  oxide,  which  occurs  in  Nature  in  the  form  of  various 
minerals  which  can  be  made  into  pigments  by  mechanical 
treatment.  In  many  cases,  this  treatment  has  already 
been  carried  out  by  Nature,  and  deposits  of  iron  oxide 
are  found  in  which  the  material  has  only  to  be  incor- 
porated with  a  vehicle  to  make  it  fit  for  immediate 
use  as  a  painters'  colour. 

Moreover,  the  same  oxide  is  obtained,  in  large  quan- 
tities, as  a  by-product  of  the  treatment  of  other  minerals. 
From  the  point  of  view  of  chemical  composition,  this 
by-product  is  of  very  low  value,  by  reason  of  the  large 
supplies  of  native  oxide  available.  By  means  of  a 
very  simple  chemical  treatment,  however,  this  by- 
product oxide  can  be  considerably  improved  in  com- 
mercial value,  being,  in  many  cases,  convertible,  by 
merely  heating  it  to  certain  temperatures,  into  a  variety 
of  colours  which  sell  at  remunerative  prices. 

Consequently,  in  view  of  the  present  condition  of 
the  chemical  industry,  the  term  "  earth  colours " 
can  be  enlarged  to  include  a  number  of  waste  products 
which  fetch  good  prices  as  colours,  though  otherwise 
practically  valueless  in  themselves. 

The  number  of  earth  pigments  is  very  large,  and 
comprises  representatives  of  all  the  principal  colours. 
For  painting  purposes,  few  pigments  beyond  the  earth 
colours  were  known  to  the  ancients ;  and  most  of  the 
colours  in  the  paintings  which  have  come  down  to  us 
from  antiquity  are  pure  earth  pigments,  thus  affording 
proof  of  their  great  durability,  having  retained  their 
freshness  unimpaired  for  hundreds — and  some  for 
thousands — of  years. 


The  earth  colours  might  be  divided  into  such  as 
occur  ready-formed  in "  Nature,  and  require  only 
mechanical  preparation,  and  which  either  require 
special  treatment  (e.  g.  calcining),  or  are  artificial 
products  (like  the  iron  oxide  mentioned  above).  Since, 
however,  such  a  classification  would  not  advantage 
our  knowledge  of  the  nature  of  this  class  of  colours, 
it  appears  useless  and  superfluous,  and  we  will  therefore 
simply  confine  ourselves  to  arranging  the  earth  pig- 
ments according  to  their  colour — white,  yellow,  red,  etc. 

Adopting  this  classification,  the  following  minerals 
and  chemical  products  may  be  considered  as  earth 
colours  :— 

White. — These  include  the  varieties  of  calcium 
carbonate,  such  as  chalk,  marble,  precipitated  chalk, 
calcium  phosphate,  calcium  sulphate  (in  the  form  of 
gypsum,  alabaster,  muriacite  and  the  precipitated 
gypsum  produced  as  a  by-product  in  many  chemical 
works),  heavy  spar,  the  different  varieties  of  clay,  and 

Yellow. — This  group  comprises  ferric  hydroxide 
(hydrated  oxide  of  iron)  in  the  form  of  the  various 
minerals  known  as  ochre ;  all  the  preparations  chiefly 
composed  of  this  hydroxide,  and  all  those  prepared 
by  artificial  means.  A  very  important  member  of  this 
group  is  orpiment ;  the  other  arsenical  compounds 
frequently  met  with  native,  being  however,  on  account 
of  their  poisonous  properties,  no  longer  used  as  pigments. 

Red. — Chief  among  the  red  earth  colours  are  those 
consisting  of  ferric  oxide  (iron  oxide),  under  various 
names.  The  only  other  member  of  the  group  is  the 
far  rarer  vermilion. 

Blue. — The  blue  earth  colours  are  few  in  number 
and  of  no  particular  beauty ;  but  they  are  of  importance 


on  account  of  their  cheapness  and  because  all  the 
artificial  blue  pigments  are  rather  expensive.  Two 
products  in  particular  merit  attention  in  this  con- 
nection, namely,  ultramarine,  and  the  mineral  known 
as  blue  ochre  or  blue  ironstone.  The  latter,  as  a  matter 
of  fact,  cannot  be  used  for  anything  else  than  a  painters' 
colour,  and  can  be  obtained  at  a  low  price;  whereas 
ultramarine  also  forms  a  valuable  raw  material  for 
the  recovery  of  copper,  and  is  therefore  dearer. 

Green. — This  group,  again,  contains  only  two  mem- 
bers, viz.  malachite  green  (chrysocolla),  and  the  green 
earths  (seladonite),  known  as  Verona,  etc. ,  green.  These 
occur  fairly  often  in  Nature,  and  the  green  earths  in 
particular  find  a  wide  industrial  application  by  reason 
of  their  low  price.  Malachite  green  is  very  similar, 
in  chemical  constitution,  to  ultramarine ;  and  both 
form  sources  of  copper  and  are  consequently  expensive. 

It  should  be  mentioned  that  both  ultramarine  and 
malachite  green  can  only  be  profitably  made  into 
pigments  where  the  minerals  can  be  obtained  cheaply, 
since  both  of  them  can  be  manufactured  where  arti- 
ficial pigments  are  produced,  and  are  put  on  the  market 
under  the  same  names  as  the  native  articles.  The 
very  low  price  of  the  green  earths  makes  them  highly 
popular  as  colouring  matters  in  certain  branches  of 
industry,  and  they  are  very  largely  used  by  wall-paper 

Brown. — This  is  a  large  group,  and  the  pigments 
composing  it  are  specially  distinguished  for  their  beauty 
and  depth  of  colour,  on  which  account  they  are  used 
in  the  finest  paintings.  Here,  again,  it  is  ferric  oxide, 
in  combination  with  water — and  therefore  ferric 
hydroxide — that  furnishes  a  large  number  of  the  mem- 
bers of  the  group.  Like  the  renowned  Siena  earth, 


the  artists'  colours  known  as  Vandyck  brown,  bole, 
Lemnos  earth,  umber,  etc.,  mainly  consist  of  more 
or  less  pure  ferric  hydroxide.  These  minerals  are, 
moreover,  specially  important  to  the  colour  manu- 
facturer, inasmuch  as  most  of  them  enable  a  large 
number  of  different  shades  to  be  obtained  by  a  simple 
method  of  treatment  consisting  merely  of  the  applica- 
tion of  heat  in  a  suitable  manner;  and  these  colours 
are  among  the  most  excellent  we  possess,  by  reason 
of  their  beauty  and  permanence.  Amongst  this  series 
must  also  be  classed  native  manganese  brown,  which 
chiefly  consists  of  a  mixture  of  manganese  oxide  and 
the  hydrated  peroxide  of  the  same  metal. 

Black. — There  is  really  only  one  member  of  this 
class,  which,  however,  is  frequently  used,  viz.  that 
form  of  carbon  occurring  as  hexagonal  crystals  and 
known  as  graphite.  Another  natural  black  natural 
product,  occasionally  used  as  a  painters'  colour  is 
the  so-called  black  chalk.  However,  since  black 
pigments  can  be  produced  very  cheaply  by  artificial 
means,  the  natural  colours  find  only  a  limited  applica- 
tion ;  and  only  in  one  instance  is  graphite  used  alone, 
viz.  for  making  blacklead  pencils. 

As  already  mentioned,  certain  chemical  industries 
furnish  by-products  which  are  of  very  little  value 
in  themselves,  and  many  of  them,  indeed,  may  be 
classed  as  worthless,  since  chemical  manufacturers 
naturally  endeavour  to  get  everything  possible  out 
of  their  materials  in  the  course  of  manufacture. 

Some  of  these  by-products,  however,  can  advantage- 
ously be  used  as  pigments,  a  good  example  of  this 
being  afforded  by  the  iron  oxide  formed  as  a  by-product 
in  the  manufacture  of  fuming  sulphuric  acid  (Nord- 
hausen  oil  of  vitriol),  by  the  old  process,  from  green 


vitriol  (ferrous  sulphate).  In  itself,  this  oxide  is 
practically  valueless,  but,  by  very  simple  treatment, 
it  can  be  converted  into  very  valuable  pigments  which 
have  a  market  value  far  in  excess  of  the  original  material. 
Although  it  has  hitherto  been  the  custom  to  confine 
the  term  earth  colours  to  such  as  occur  ready-formed 
in  Nature  and  only  require  simple  mechanical  treatment 
to  make  them  ready  for  immediate  use  as  pigments, 
the  author  is  nevertheless  of  opinion  that  a  book  dealing 
exhaustively  with  earth  colours  should  also  make  some 
mention  of  all  the  mineral  colouring  matters  which 
can  be  easily  made  into  pigments  by  simple  processes, 
such  as  calcination  or  bringing  into  association  with 
other  substances.  In  accordance  with  this  view,  the 
present  work  will  describe  all  the  pigments  that  are 
obtainable  in  this  manner.  Most  of  the  earth  colours 
consist  of  decomposition  products  of  certain  minerals ; 
and  this  applies  particularly  to  such  of  them  as  contain 
iron  oxide.  According  as  the  decomposition  of  the 
original  mineral  has  been  more  or  less  extensive,  the 
natural  product  exhibits  different  properties;  and 
the  manufacturer  must  consequently  endeavour  to 
treat  them  in  such  a  manner  as  to  ensure  that  the  pig- 
ment obtained  will  be  as  uniform  as  possible  in  shade 
and  permanence.  In  order  to  accomplish  this  it  is 
essential  to  have  an  accurate  knowledge  of  the  origin 
of  the  raw  material  under  treatment,  and  of  its  chemical 
and  physical  properties.  In  view  of  this,  the  author 
considers  it  necessary  to  deal  more  fully  with  the  pig- 
mentary earths  forming  the  raw  materials  of  the  earth 
colours,  before  passing  on  to  the  preparation  of  the 
colours  themselves. 



THE  minerals  constituting  the  raw  materials  for  the 
preparation  of  the  earth  colours  occur  under  very 
divergent  conditions  in  Nature.  Some  of  them,  such 
as  chalk,  form  immense  deposits,  even  whole  mountains, 
whilst  in  other  cases,  e.  g.  the  blue  ferruginous  earths, 
the  occurrence  is  connected  with  certain  local  con- 
ditions, and  many  are  found  only  in  isolated  deposits, 
as  pockets  or  beds.  This  last  is  the  case,  for  instance, 
with  the  handsome  brown  iron  pigments ;  and  indeed 
the  names  by  which  they  are  known  indicate  that  they 
are  only  found  in  well-defined  localities,  or  that  they 
are  met  with  of  special  quality  there.  The  brown 
earth  colour  known  to  all  painters  as  Terra  di  Siena, 
is  found  at  many  other  places  as  well  as  near  Siena, 
but  the  product  from  that  city  acquired  aforetime 
a  special  reputation  for  beauty,  and  therefore  all 
similar  earths,  provided  they  are  equal  to  that  from 
Siena,  also  bear  the  same  name  in  commerce. 

A  number  of  raw  materials  for  the  preparation  of 
earth  colours  are  found,  it  is  true,  in  many  deposits, 
but  their  utilisation  depends,  in  turn,  on  local  con- 
ditions. For  example,  many  copper  mines  contain, 
in  addition  to  the  other  cupriferous  minerals,  those 
used,  in  the  powdered  state,  as  ultramarine  or  ultra- 
marine green,  and  not  infrequently  lumps  of  mineral 


are  found  containing  both  blue  and  green  together. 
However,  it  is  only  when  these  minerals  occur  in 
sufficient  quantity  to  make  the  necessary  sorting 
profitable  that  their  manufacture  into  pigments  can 
be  regarded  as  practicable. 

Before  commencing  to  work  a  deposit  it  is  essential 
to  make  sure  whether  the  raw  material,  or  pigmentary 
earth,  is  actually  suitable  for  the  manufacture  of  earth 
colour.  Even  the  general  character  of  the  material 
is  important,  those  of  soft,  earthy  consistency  being 
much  easier  to  treat,  and  the  cost  of  preparation 
smaller,  than  if  the  raw  material  be  hard,  tough  and 

The  extent  and  thickness  of  the  deposit,  and  the 
ease  with  which  it  can  be  worked,  also  play  an  im- 
portant, and  even  decisive  part,  since,  other  conditions 
being  equal,  it  will  not  pay  to  erect  a  colour  works 
unless  the  raw  material  is  available  in  sufficient  quantity 
and  is  cheap.  Generally,  the  deposit  is  not  homo- 
geneous throughout,  the  mineral  being  purer  in  some 
places  and  more  contaminated  with  gangue  in  others. 
The  percentage  of  moisture  also  varies,  and  in  short, 
a  number  of  circumstances  must  be  taken  into  con- 
sideration in  forming  a  conclusion  as  to  whether  a 
deposit  is  workable  or  not. 

In  order  to  arrive  at  a  reliable  opinion  on  all  these 
conditions,  sampling  is  indispensable.  If  the  samples 
are  of  uniform  character,  they  can  be  mixed  together 
to  make  an  average  sample.  But  if  they  differ  con- 
siderably in  appearance,  general  character,  proportion 
of  gangue,  etc.,  it  is  preferable  to  examine  them 
separately,  more  especially  when  the  area  which  each 
represents  is  large. 
The  examination  should  extend,  on  the  one  hand, 


to  the  natural  percentage  of  moisture,  and,  on  the 
other,  to  the  purity  of  the  material.  The  water  con- 
tent is  determined  by  thoroughly  drying  a  weighed 
sample,  bearing,  however,  in  mind  the  fact  that 
pigmentary  earths  of  a  clayey  nature  vary  in  water 
content  according  to  the  time  of  year,  besides  changing 
in  accordance  with  the  weather  when  the  won  material 
is  stored  in  the  open. 

The  purity  can  only  be  ascertained  by  an  examination 
in  which  a  sample  of  the  soft,  clayey  material  is  crushed 
and  passed  through  a  narrow-mesh  gauze  sieve,  the 
amount  of  the  coarse  particles — sand,  small  stones, 
etc. — remaining  on  the  sieve  being  determined.  A 
more  accurate  method,  of  course,  is  to  separate  the 
true  pigmentary  earth  from  the  gangue  by  levigation. 
For  this  purpose,  a  weighed  quantity  of  the  crushed, 
air-dry  sample  is  placed  in  a  relatively  narrow  glass 
vessel  and  thoroughly  mixed  with  water,  the  turbid 
supernatant  liquid  being  poured  off  after  a  short 
interval.  The  residue  is  repeatedly  treated  in  the 
same  way,  until  no  more  fine  particles  remain  in 
suspension,  the  residue  then  consisting  of  impurities, 
or  gangue.  Of  course,  the  washings  can  be  collected, 
the  suspended  matter  allowed  to  settle,  and  finally 
weighed  in  an  air-dry  condition.  By  this  means  an 
approximate  idea  of  the  yield  of  earth  colour  can  be 
obtained  at  the  same  time. 

Raw  materials  which  are  not  amorphous,  soft  and 
clayey  must  first  be  crushed,  an  operation  facilitated 
by  heating  to  redness  and  quenching  in  cold  water. 
Oftentimes  the  heating  causes  a  change  of  colour 
and  improves  the  covering  power — a  point  to  which 
reference  will  be  made  later  on. 

In   the   following   description   of   the   various   raw 


materials,  the  chemical  composition  of  the  pure 
minerals  will  be  given,  together  with  an  enumeration 
of  the  most  common  impurities. 


Limestone  (Calcite,  Limestone,  Chalk) 

The  number  of  materials  furnishing  white  earth 
colours  is  comparatively  large,  and  these  colours  are 
particularly  important,  because,  not  only  are  they 
extensively  used  by  themselves,  but  they  also  serve 
as  adjuncts  to  other  colours  and  for  the  production 
of  special  shades.  The  chief  raw  material  for  the 
preparation  of  white  earth  colours  is  the  mineral 
calcite  in  its  numerous  modifications. 

Calcite,  or  calc  spar,  occurs  very  frequently  in 
Nature,  and  is  one  of  the  most  highly  diversified 
minerals  known.  In  its  purest  state  it  appears  as 
"  double  spar  "  (calcite),  in  the  form  of  water- white 
crystals,  which  are  very  remarkable,  for  certain  optical 
properties.  White  marble  is  also  a  very  pure  variety 
of  calcite,  in  which  the  individual  crystals  are  very 
small.  The  various  coloured  marbles  owe  their  appear- 
ance to  certain  admixtures  of  extraneous  substances, 
chiefly  metallic  oxides. 

No  sharp  line  of  demarcation  separates  marble  from 
ordinary  limestone,  the  difference  between  them 
really  consisting  only  in  the  degree  of  fineness  of 
grain ;  and  all  limestones  which  grind  and  polish  well 
may  be  classed  as  marble.  As  is  the  case  with  marble, 
there  are  also  limestones  of  various  colours,  grey  being, 
however,  the  most  common.  This  grey  limestone 
forms  huge  mountain  masses  which,  in  Europe,  follow 


for  example,  the  Alpine  chain  on  its  northern  and 
southern  edges. 

A  few  other  examples  of  calcite  may  be  mentioned 
which  occur  in  certain  localities  and,  in  part,  are  still 
in  course  of  formation.  To  these  belong  the  stalactites 
and  stalagmites,  which  sometimes  consist  of  extremely 
pure  calcite.  They  are  formed  by  the  action  of  water, 
containing  carbonic  acid  in  solution,  which  trickles 
through  cracks  and  cavities  in  limestone  rock  and 
dissolves  out  calcium  carbonate  from  the  adjacent 
stone.  On  prolonged  exposure  to  the  air  such  water 
gives  off  its  free  carbonic  acid  again  ;  and  as  the  calcium 
carbonate  is  insoluble  in  pure  water,  it  separates  out 
in  crystalline  form.  The  masses  formed  in  this  way 
usually  resemble  icicles  in  shape,  and  the  finest 
examples  are  to  be  found  in  the  well-known  stalactite 
grottoes  at  Krain,  whilst  the  grotto  at  Adelsberg  is 
renowned  for  its  beautiful  stalactites.  Occasionally, 
stalactites  have  an  opaque  yellow  or  brownish  tinge, 
which  they  owe  to  the  presence  of  iron  oxide. 

A  formation  similar  in  its  origin  to  stalactites  is  the 
so-called  calc  sinter  and  calcareous  tuff.  The  former 
often  occurs  in  cavities  as  irregular  masses  which,  in 
some  places,  enclose  large  quantities  of  fossil  animal 
bones,  in  which  case  they  form  "  bone  breccia " 
(crag  breccia).  Calcareous  tuff  is  deposited  from 
numerous  springs,  occasionally  in  very  large  quantities, 
enveloping  plants  and  sometimes  forming  thick  deposits 
in  which  the  structure  of  the  plants  can  be  clearly 

In  some  places  a  more  or  less  pure  white,  extremely 
friable  variety  of  calcite  is  met  with  under  the  name 
"mountain  milk"  or  "mountain  chalk"  (earthy 
calcite),  which  seems  to  be  a  decomposition  product, 


and  consists  of  a  mixture  of  arragonite  and  chalk. 
Arragonite — which  will  be  referred  to  later — is  com- 
pletely identical,  chemically,  with  calcite — both  being 
composed  of  calcium  carbonate — the  sole  difference 
being  their  crystalline  form. 

The  most  important  for  the  colour-maker,  however, 
is  the  variety  known  as  chalk.  This  is  really  a  fossil 
product,  i.  e.  it  consists  of  the  microscopic  shells  of 
marine  animals  united  into  solid  masses.  Despite 
the  smallness  of  these  animals,  their  epoch  lasted  long 
enough  for  their  shells  to  form  entire  mountains  which 
are  encountered  all  over  the  world.  A  large  part  of 
the  coast  of  England,  the  island  of  Riigen,  and  many 
other  localities,  consist  entirely  of  chalk. 

In  many  cases,  chalk  is  found  interspersed  with 
nodular  masses  of  flint,  and  in  some  places  it  also 
contains  great  quantities  of  the  remains  of  other 
marine  animals,  such  as  sea  urchins,  the  spines  of  which 
occur  in  such  numbers  in  certain  kinds  of  chalk  as  to 
unfit  them  entirely  for  use  as  a  pigment. 

The  foregoing  varieties  of  calc  spar  are  the  most 
important,  and  also  occur  in  large  quantities ;  but,  to 
complete  the  tale,  it  is  necessary  to  mention  also  a  few 
others  which,  however,  are  only  found  in  small  amounts. 
To  these  belong,  for  example,  anthracolite,  a  limestone 
stained  quite  black  by  coal ;  the  oolithic  limestones  or 
roe  stones,  which  are  composed  of  granules  resembling 
fish  roe ;  muschelkalk,  which  is  also  of  fossil  character 
and  is  almost  entirely  composed  of  mussel  shells 
cemented  together  with  lime ;  the  marls,  which  consist 
of  calc  spar  mixed  with  varying  quantities  of  clay  and 
consequently  often  bear  a  great  resemblance  to  loam 
in  their  properties.  A  few  of  these  varieties  find 
extensive  employment  for  certain  purposes,  some 


marls  for  instance  being  used  for  making  hydraulic 
lime,  whilst  all  modifications  of  calc  spar  that  are 
sufficiently  pure  can  be  burned  for  quick  lime. 

It  has  already  been  stated  that  the  mineral  arragonite 
is.  identical,  chemically,  with  calc  spar,  since  both 
consist  of  calcium  carbonate,  but  differ  in  their  crystal- 
line habit.  Thus,  whereas  the  crystals  of  calc  spar 
belong  to  the  rhombohedral  or  hexagonal  system,  those 
of  arragonite  are  always  rhombic.  This  occurrence  of 
one  and  the  same  substance  in  two  different  crystalline 
forms  is  known  as  dimorphism,  and  calcium  carbonate 
is  therefore  dimorphous.  Whether  calcium  carbonate 
assumes  the  form  of  calcite  or  arragonite  depends 
entirely  on  physical  causes.  When  the  deposition  of 
the  carbonate  takes  place  from  a  cold  solution  the 
shape  of  the  crystals  is  always  one  belonging  to  the 
hexagonal  or  rhombohedral  system;  but  when  it 
is  from  hot  solution,  rhombic  crystals  are  invariably 
formed,  calc  spar  resulting  in  the  former  case  and 
arragonite  in  the  latter. 

These  different  methods  of  formation  which  can  be 
carried  out  in  the  laboratory  by  producing  the  re- 
quisite conditions,  occur  on  the  large  scale  in  many 
parts  of  the  world.  Wherever  a  hot  spring  comes  to 
the  surface,  containing  considerable  amounts  of  lime 
in  solution,  this  separates  out  in  the  form  of  arragonite, 
which  received  its  name  from  the  circumstance  that 
specially  handsome  crystals  of  this  mineral  are  found 
in  Arragon. 

One  of  the  best-known  places  where  the  formation 
of  arragonite  can  be  observed  at  the  present  time  is 
Carlsbad  in  Bohemia.  The  hot  springs  there  deposit 
a  very  large  amount  of  lime,  which  is  stained  more 
or  less  yellow  or  red  by  the  presence  of  varying  quan- 


titles  of  iron  oxide,  and,  under  the  name  of"  sprudel- 
stein  "  is  used  for  producing  various  works  of  art. 
When  the  hot  springs  bring  up  particles  of  sand,  the 
lime  substance  incrusts  these  sand  grains,  forming 
globular  masses  resembling  peas,  and  consequently 
named  pisolite. 

In  chemical  composition,  calcite  and  arragonite 
consist  of  a  combination  of  calcium  oxide  (lime)  and 
carbonic  acid,  the  formula  being  expressed  by  CaCO3. 
Calcium  carbonate  is  insoluble  in  pure  water,  but 
dissolves  somewhat  freely  in  water  charged  with  free 
carbonic  acid.  It  is  assumed  that  a  compound  is 
formed,  which  is  known  as  calcium  bi-  (or  acid)  car- 
bonate, is  very  unstable  and  can  only  exist  in  a  state 
of  solution.  When  a  solution  of  calcium  bicarbonate — 
which  can  be  prepared  by  passing  carbonic  acid  gas 
through  water  containing  finely  divided  calcium 
carbonate  in  suspension — is  exposed  for  some  time  to 
the  air,  it  soon  becomes  cloudy,  and  a  deposit  of  calcium 
carbonate  settles  down  at  the  bottom  of  the  vessel, 
because,  in  the  air  the  dissolved  calcium  bicarbonate 
is  decomposed  into  free  carbonic  acid  gas  and  calcium 
carbonate,  which  latter,  as  has  been  mentioned,  is 
quite  insoluble  in  water.  It  has  already  been  stated 
that  this  phenomenon  goes  on  in  Nature  in  the  forma- 
tion of  stalactites,  lime  sinter  and  calcareous  tuff. 

Calcium  carbonate  is  readily  soluble  in  acids,  the 
contained  carbonic  acid  being  liberated  (as  carbon 
dioxide)  with  effervescence.  WTien  such  acids  are 
employed  for  solution  as  form  readily  soluble  salts 
with  lime,  such  as  hydrochloric,  nitric,  acetic,  etc. 
acids,  a  perfectly  clear  solution  is  obtained;  but  if 
sulphuric  acid  is  used,  a  white  pulpy  mass  is  formed, 
consisting  of  calcium  sulphate,  or  gypsum,  which, 


owing  to  its  low  solubility,  separates  out  as  small 
crystals.  Any  sandy  residue  left  when  calcium 
carbonate  is  dissolved,  mostly  consists  of  quartz  sand. 
In  dissolving  dark -coloured  limestones,  grey,  or  even 
black,  flakes  are  left,  which  consist  of  organic  material 
very  high  in  carbon.  On  limestone  being  subjected 
to  fairly  strong  calcination,  all  the  carbonic  acid  is 
expelled,  leaving  behind  the  so-called  quick  or  burnt 
lime,  which  is,  chemically,  calcium  oxide  :— 

CaCO3      =      CaO      +      CO2 

Calcium  Quick  Carbon 

carbonate  lime  dioxide 

If  burnt  lime  be  left  exposed  to  the  air  for  some 
time,  it  again  gradually  absorbs  carbon  dioxide  and 
is  reconverted  into  calcium  carbonate.  When  burnt 
lime  is  sprinkled  with  water  it  takes  up  the  latter 
avidly,  becoming  very  hot  and  finally  crumbling  down 
to  a  very  friable  white  powder,  consisting  of  slaked 
or  hydrated  lime  (calcium  hydroxide,  Ca(OH)2).  The 
considerable  rise  of  temperature  in  quenching  the  lime 
is  due  to  the  chemical  combination  of  the  calcium  oxide 
and  water. 

Both  quick  and  slaked  lime  dissolve  to  a  certain 
extent  in  water,  and  impart  strongly  alkaline  properties 
thereto,  lime  being  one  of  the  strongest  of  bases.  On 
exposure  to  the  air,  the  solution  of  quick  lime  in 
water  (lime-water)  quickly  forms  an  opalescent  super- 
ficial film  of  calcium  carbonate,  and  in  a  short  time  no 
more  lime  is  present  in  solution,  the  whole  having  been 
transformed  into  calcium  carbonate,  which  settles 
down  to  the  bottom  of  the  vessel  as  a  very  fine  powder. 

Limestone  that  consists  entirely  of  calcium  oxide 
and  carbon  dioxide  is  of  rare  occurrence  in  Nature, 


foreign  substances  being  nearly  always  present.  Since 
the  nature  of  these  admixtures  is  of  the  greatest 
importance  to  the  colour-maker,  owing  to  the  consider- 
able influence  they  exert  on  the  suitability  of  the 
minerals  for  his  purposes,  it  is  necessary  that  these 
extraneous  substances  occurring  in  limestone  should 
be  more  closely  described. 

Nearly  all  varieties  of  limestone  contain  certain 
proportions  of  ferrous  and  ferric  oxides.  The  presence 
of  ferrous  oxide,  when  the  relative  amount  is  but 
small,  cannot  be  detected  by  mere  inspection ;  and 
even  many  limestones  containing  really  appreciable 
quantities  of  ferrous  oxide  are  pure  white  in  colour 
so  long  as  they  are  in  large  lumps.  If,  however,  such 
a  limestone  be  reduced  to  powder  and  exposed  to  the 
air  for  a  short  time,  it  gradually  assumes  a  yellow 
tinge,  the  depth  of  which  increases  with  the  length 
of  exposure. 

The  cause  of  this  change  is  due  to  the  fact  that 
ferrous  oxide  has  a  great  affinity  for  oxygen,  by  absorb- 
ing which  it  changes  into  ferric  oxide.  (Ferrous  oxide 
consists  of  FeO,  ferric  oxide  of  Fe2O3.)  Ferrous  oxide 
and  its  compounds  are  of  a  pale  green  colour  which 
is  not  very  noticeable,  whereas  ferric  oxide  has  a  very 
powerful  yellow  colour,  and  consequently  the  lime- 
stone, when  its  superficial  area  has  been  greatly 
increased  by  reduction  to  powder,  assumes  the  yellow 
tinge  due  to  ferric  oxide.  A  limestone  exhibiting 
this  property  can  evidently  not  be  used  for  making 
white  earth  colours,  but  is,  at  best,  only  suitable  for 
mixing  with  other  colours. 

Occasionally,  limestone  contains  varying  quantities 
of  magnesia,  and  when  this  oxide  is  present  in  large 
amount,  changes  into  another  mineral  known  as 



dolomite.  In  many  places  this  dolomite  forms  large 
masses  of  rock,  which,  however,  is  not  employed  for 
making  colours,  owing  to  the  yellow  shade  imparted 
by  the  fairly  large  amount  of  ferric  oxide  present. 

Gypsum  (Alabaster) 

This  mineral  occurs  native  in  many  places,  and  is 
frequently  worked  for  a  number  of  purposes.  Gypsum 
occurs  in  Nature  in  a  great  variety  of  forms.  The 
purest  kind  is  met  with  either  as  water-clear  crystals, 
which  cleave  readily  in  two  directions,  or  as  trans- 
parent tabular  masses  (selenite)  which  also  cleave  easily. 
Micro-crystalline  fine-grained  gypsum  is  milk-white  in 
colour,  highly  translucent  and  is  largely  used,  under 
the  name  of  alabaster,  in  sculpture.  Owing  to  its 
low  hardness,  alabaster  can  be  readily  cut  with  a  knife, 
and  on  this  account  is  frequently  shaped  by  planing  or 

Gypsum  is  generally  met  with  in  dense  masses, 
which  may  be  of  any  colour,  grey,  blue  and  reddish 
shades  being  the  most  common,  whilst  pure  white  is 
rarer.  The  dark-coloured  varieties  can  only  be  used 
for  manurial  purposes ;  but  the  white  finds  a  two-fold 
application  as  a  pigment,  and,  in  the  calcined  state, 
for  making  plaster  casts. 

In  point  of  chemical  composition,  gypsum  consists  of 
sulphate  of  lime,  or  calcium  sulphate  (CaSO4  -f  2H2O). 
It  is  soluble  in  water,  but  only  in  such  small  quantity 
that  over  400  parts  of  the  latter  are  needed  to  dissolve 
one  part  of  gypsum.  On  being  heated  to  between 
120°  and  130°  C.,  gypsum  parts  with  its  two  molecules 
of  combined  water  and  becomes  anhydrous  calcium 
sulphate  or  burnt  gypsum.  When  this 'latter  is  stirred 
with  water  to  a  pulp,  it  takes  up  the  water  again,  with 


considerable  evolution  of  heat,  swelling  up  considerably 
and  setting  quickly  to  a  solid  mass. 

The  number  of  substances  exhibiting  this  property 
being  small,  burnt  gypsum  is  very  frequently  used  for 
making  casts  of  statuary,  and  for  stucco  work  in 
building.  Finely  ground  white  gypsum  can  also  be 
used  as  a  pigment,  but  is  inferior  to  calcium  carbonate 
in  covering  power,  and  is  therefore  seldom  employed 
for  this  purpose,  though  frequently  added  to  other 
colours.  The  mineral  known  as  muriacite  or  anhydrite 
consists  of  anhydrous  calcium  sulphate ;  and  is  there- 
fore similar  in  composition  to  burnt  gypsum;  but  it 
lacks  the  property  of  combining  with  water  when 
brought  into  contact  therewith. 

Barytes,  or  Heavy  Spar 

The  mineral  known  as  heavy  spar  occurs  in  very 
large  quantities  and  in  numerous  localities.  It  forms 
rhombic  crystals,  which  are  very  often  extremely 
well  developed  and  form  flat  plates  of  considerable 
size.  A  remarkable  peculiarity  of  this  mineral  is  its 
high  specific  gravity,  which  is  due  to  the  barium 
content.  It  is  found  native  in  all  colours,  white  being 
the  most  common. 

Chemically,  heavy  spar  is  barium  sulphate,  BaS04. 
It  can  be  used  as  a  pigment  per  se,  but  only  when 
prepared  artificially,  the  trade  name  for  the  product 
being  permanent  white,  or  blanc  fixe.  Powdered 
native  heavy  spar,  even  when  ground  ever  so  fine,  has 
not  enough  covering  power,  this  property  being 
peculiar  to  the  artificial  product. 

When  it  is  desired  to  mix  other  pigments  with  a 
white  substance,  to  lighten  the  shade,  permanent 
white  can  be  specially  recommended,  since  it  is  quite 


insensitive  to  atmospheric  influences  and  has  no 
chemical  action  on  the  colour,  so  that  it  can  be  used 
with  even  the  most  delicate  colours  without  risk.  In 
this  way,  not  only  can  the  colours  be  considerably 
cheapened,  but  over -dark  colours  can  be  shaded  to 
the  desired  extent.  Another  advantage  of  such 
mixtures  is  that  a  smaller  quantity  of  oil  or  varnish 
is  required,  barytes  only  needing  about  8%  of  its  own 
weight  of  vehicle  to  form  a  workable  mixture,  whilst 
other  pigments  take  five  times  as  much,  or  even  more. 
In  many  cases  the  low  covering  power  of  barytes 
enables  large  quantities  to  be  added,  and  this  reacts 
favourably  on  the  consumption  of  varnish. 

Another  barium  mineral  is  witherite,  or  barium 
carbonate.  This  is  not  used  direct  as  a  pigment, 
but — in  contrast  to  heavy  spar — is  readily  soluble  in 
hydrochloric  acid,  and  therefore  serves  as  raw  material 
for  the  preparation  of  artificial  barytes  and  other 
barium  compounds,  the  first -named  being  obtained 
by  treating  a  solution  of  barium  chloride  with  sulphuric 
acid,  insoluble  barium  sulphate  being  precipitated. 

Talc,  Soap  stone,  Steatite 

Talc  occurs  in  Nature  either  as  a  pure  white'  mass, 
of  greasy  lustre,  or  occasionally  as  yellow,  green  or 
grey  masses,  all  distinguished  by  a  peculiar  greasy 
appearance  and  a  soapy  feel.  This  appearance  is 
common  to  all  the  minerals  of  the  steatite  group,  and 
is  the  cause  of  their  generic  name,  soapstone.  Although 
the  steatites  have  a  very  low  degree  of  hardness — 
most  of  them  can  be  scratched  by  the  finger-nail- 
some  difficulty  is  encountered  in  reducing  them  to 
fine  powder.  Calcination  usually  increases  the  hard- 
ness considerably,  so  that,  in  some  cases,  the  calcined 


mineral  gives  off  sparks  when  struck  with  a  steel 

Soapstone  is  composed  of  magnesium  silicates, 
containing  varying  proportions  of  magnesia  and  silica, 
together  with  a  small  quantity  of  water,  apparently  in 
a  state  of  chemical  combination,  a  very  high  tempera- 
ture, approaching  white  heat,  being  required  to  effect 
its  complete  expulsion,  the  residue  then  attaining  the 
aforesaid  high  degree  of  hardness.  The  composition 
of  talc  can  be  expressed  by  the  symbol  H2Mg2(SiO3)4, 
corresponding  to  63-52%  of  silica,  31*72%  of  magnesia, 
and  4*76%  of  water.  In  some  varieties  of  talc,  a 
portion  (1-5%)  of  the  magnesia  is  replaced  by  ferrous 
oxide.  Talc  is  quite  unaffected  by  the  action  of 
dilute  acids,  boiling  concentrated  sulphuric  acid  being 
required  to  decompose  it,  with  separation  of  silica. 

Owing  to  its  low  specific  gravity  and  chemical 
indifference,  talc  is  suitable  for  lightening  the  shade  of 
certain  lake  pigments.  It  can  also  be  used  as  a  pig- 
ment by  itself,  and  also  as  a  gloss  on  wall-paper,  for 
mixing  with  paper  pulp,  and  for  various  other  purposes. 


The  mineral  known  as  clay  is,  in  all  cases,  a  product 
of  the  decomposition  of  other  minerals,  mainly  felspar. 
This  substance  is  a  double  silicate  of  alumina  and 
potash,  K2O.Al2O8.(SiO2)6.  Pure  kaolin  is  Al2O3(SiO2)2 
+  2H2O,  or  46-50%  silica,  39-56%  alumina,  13-9% 

Clay  may  be  supposed  to  have  been  formed  by  the 
conversion  of  felspar,  under  the  action  of  air  and  water, 
into  silicate  of  alumina,  the  silicate  of  potash  being 
dissolved  out.  Being  insoluble,  the  silicate  of  alumina 
would  be  transported  by  the  water,  in  a  very  fine 


state  of  division,  and  finally  deposited  as  a  sediment, 
which  in  course  of  time  became  a  solid  mass.  This, 
when  again  brought  into  contact  with  water,  forms  a 
very  plastic  pulp  which,  when  dried  and  baked,  forms 
a  solid  mass,  brick,  which  is  no  longer  affected  by 
water.  Perfectly  pure  clay  forms  a  white  mass, 
which,  under  the  name  of  China  clay  or  kaolin,  is  used 
for  making  porcelain,  and  is  only  occasionally  met 
with  in  large  quantities. 

Pure  kaolin  is  characterised  by  its  great  chemical 
indifference,  being  decomposed  only  by  strong  alkalis 
and  sulphuric  acid.  At  the  high  temperature  of  the 
pottery  kiln,  kaolin  sinters  to  a  very  compact  mass, 
but  cannot  be  fused,  except  when  small  quantities  are 
subjected  to  the  intense  heat  of  the  oxyhydrogen 
flame,  whereupon  it  fuses  to  a  colourless  glass  of  great 

In  an  impure  state,  silicate  of  alumina  occurs 
frequently  in  Nature,  and  then  forms  the  minerals 
known  under  the  generic  names  of  clay,  loam,  marl, 
etc.  These  impure  clays  contain  varying  proportions 
of  extraneous  minerals  which  produce  changes  in  the 
physical  and  chemical  properties.  They  are  grey, 
blue  or  yellow  in  colour,  the  grey  and  blue  varieties 
mostly  containing  appreciable  quantities  of  ferrous 
oxide,  whilst  the  yellow  kinds  contain  ferric  oxide. 
When  fired,  all  of  them  become  yellow  or  red,  the 
ferrous  oxide  being  transformed  into  ferric  oxide  by 
the  heat.  Some  fairly  white  clays  are  high  in  lime, 
which  makes  them  fusible  at  high  temperatures.  In 
some  very  impure  kinds,  even  the  comparatively  low 
heat  of  the  brick-kiln  is  sufficient  to  cause  partial 
fusion.  For  colour-making,  the  white  clays,  especially 
kaolin  and  pipeclay,  form  a  highly  important  material, 


being  procurable  at  very  low  prices  and  fairly  easy  to 

The  white  clays  aie  either  used  as  pigments  by 
themselves,  or  for  mixing  with  other  colours  of  low 
specific  gravity. 


The  number  of  yellow  earths  is  large,  but  most  of 
them  exhibit  a  certain  similarity  in  chemical  composi- 
tion, the  pigmentary  principle  in  the  majority  being 
either  ferric  oxide  or  ferric  hydroxide.  The  former  is 
yellow,  the  latter  brown,  and  the  colour  of  the  minerals 
resembles  that  of  the  preponderating  iron  compound. 

Brown  Ironstone 

The  mineral  known  as  brown  ironstone  consists  of 
ferric  hydroxide,  and  usually  forms  compact  masses, 
no  decided  crystals  having,  so  far,  been  observed. 
The  lumps  have  an  irregular  or  earthy  fracture,  a 
hardness  of  5-5*5,  and  a  sp.  gr.  between  3*40  and  3-95. 
The  colour  ranges,  in  the  different  varieties,  from 
yellowish  (rusty)  brown,  through  cinnamon  to  blackish- 
brown.  The  chemical  composition  of  the  pure  lumps 
may  be  expressed  by  the  symbol  2Fe2O3  -j-  3H2O; 
but  a  little  manganese  oxide  and  silica  is  generally 
present  even  in  the  pure  kinds. 

The  chief  varieties  of  this  mineral  are  : — 

(a)  Fibrous   brown   iron   ore,    or   brown   hematite, 
mostly  forming  reniform  or  stalactitic  masses. 

(b)  Compact    brown    ironstone,    usually    in    dense 
masses,  and  not  infrequently  also  appearing  in  pseudo- 
morphs  of  other  minerals. 

(c)  Ochreous  brown  ironstone.     This  variety  is  the 
most  important  to  the  colour-maker,  for  whose  purposes 


it  is  preferably  used.  It  nearly  always  forms  very 
loose,  earthy  masses,  yellow  or  brown  in  colour. 

(d)  Clay  ironstone.  This  consists  of  a  mixture  of 
the  above-mentioned  varieties  with  variable  propor- 
tions of  other  minerals,  clay  being  the  most  common 
ingredient.  Nodular  iron  ore,  oolitic,  bog  and  siliceous 
ore  belong  to  this  class,  as  also  the  minette  ores  that 
are  found  in  great  abundance  in  Alsace-Lorraine, 
Belgium  and  Luxemburg,  and  are  classed  with  the 
oolitic  brown  ironstones. 

In  most  cases,  the  varieties  enumerated  are  found 
together,  and  are  used  for  the  production  of  iron. 
The  ochre  constituting  the  most  interesting  member 
to  the  colour-maker  often  occurs  as  deposits  embedded 
in  dense  masses  of  brown  ironstone,  though  in  many 
places  it  is  found  by  itself. 


The  following  analyses  of  brown  ironstone  from  different  deposits 
will  give  an  idea  of  the  composition  of  these  minerals. 

Ordinary  Brown  Ironstone 









8.          9. 


Ferric  oxide     . 


\                 j 

73-75    77-54    78-50 





Manganese  oxide 



2-70      1-95 




|     .  . 




.  — 

—        .  —  . 

33-9      37-88    54-80 



.  — 


.  — 

.  — 

•  — 


0-15      0-17      0-57 




.  — 


.  — 



10-03      0-88      1-15  ,    2-50 




0-48      5-08      3-55      2-85 

0-41      0-32      0-50      0-34 



1-25      4-50      0-18      0-90 


0-02        0-38 

Silica       . 

•  — 






33-38        O-O2  '     0-38 


.  — 

.  — 

.  — 




•  — 

"  —      i      —  ' 

P205        . 
Sulphur  . 

•  —  • 


•  — 





0-06      0-04  Trace 


.  — 



.  — 




0-56     0-02 


Loss  on  incin-   \ 
eration          / 








7-77    10-55 


Deposits:  (i)  Hamm;  (2)  Schmalkalden ;  (3)  Hiittenberg  (Carynthia) ;  (4)  Styria 
(5)  and  (9)  Bilbao;  (6)  Algeria;  (7)  Schwelm  (Westphalia);  (8)  Elbingcrode  (Harz) ; 
(10)  Pennsylvania. 


Argillaceous  Brown  Ironstone 

a.          b. 







,'.              ^ 

Ferric  oxide     . 

80-76    19-4 









—    '     — 

40-90      2I-69 

Manganese  oxide 

—        8-2 








—  •        —  — 

—  . 

—  . 

•  — 

•  —  - 

Zinc  oxide 

0-92      1-6 





.  — 

.  —  . 


2-36    ii-o 







4-95      3-88 


—        2-6 







5-59    21-25 



.  —  . 



•  — 

0-49      0-30 

Silica       . 

4-58     48-61 







16-63    14-71 

PaOg        • 

~            ~ 






1-13      0-48 


.  —  -      '      






—         — 

Sulphur  . 

__      '      



.  — 



o-io      0-05 

Loss  on  incin-  \ 
eration           / 

1271  ;    9-1 







16-04    28-70 

(a)  Oolitic  (pea)  ore  from  Elligserbrick  (Brunswick) ;  (b)  from  Durlach  (Baden) ; 
(c)  and  (d)  Ore  from  Esslingen ;  (e)  Oolitic  ore  from  Siptingen  (Baden) ;  (/)  from  Adenstedt, 
nr.  Pirna  (argillaceous) ;  (g)  Ibid,  (calcareous) ;  (h)  Minette  from  Esch ;  (i)  Red  minette 
from  Dolvaux;  (k)  Brown  minette  from  Redange. 

Limonite  (Bog  Iron  Ore) 







Ferric  oxide 




67-59      7°'°5 


Manganese  oxide 


3-19             3-87 

1-45        178 


P205  . 


0-67             I'I3 

0-18        0-34 


S03     . 

3-07     Trace 

0-21     Trace 



6-00        7-00        7-15 



1  6-60 


.  —  • 





Lime  . 

.  —  . 






Magnesia     . 


—                 0'I5 




Water  and           \ 
organic  acids    / 







(i)  Limonite  from  Lausitz ;  (2)  Limonite  from   Auer,  nr.  Moriz- 
burg  ;  (3  to  6)  Swedish  limonite. 


Ochre,  or  yellow  Terra  di  Siena,  forms  earthy -looking 
masses,  fawn,  reddish -yellow  to  brownish-red  in 
colour.  Whilst  not  infrequent  in  Nature,  ochre  is 
only  found  in  small  quantities,  as  pockets,  and  not  as 
extensive  deposits.  The  discovery  of  a  bed  of  good 


coloured  ochre  is,  however,  always  a  very  valuable 
find,  bright  natural  ochres  being  somewhat  rare,  and 
most  kinds  requiring  special  preparation  before  they 
can  be  used  as  painters'  colours.  Owing  to  the  com- 
parative scarcity  of  good  coloured  ochres,  they  are 
often  called  after  the  place  of  origin,  such  as  Thuringian, 
Italian  (Siena),  English,  etc.,  ochre. 

In  nearly  every  case,  ochre  is  a  decomposition  product 
of  various  ferruginous  minerals,  which  has  been 
transported  by  water,  often  in  admixture  with  other 
minerals,  and  finally  deposited  in  the  places  where  it 
is  now  found.  Most  ochres  consist  of  varying  mixtures 
of  clay,  ferric  hydroxide  and  lime ;  and,  as  a  rule,  the 
higher  the  proportion  of  ferric  hydroxide,  the  deeper 
the  colour.  Thus,  for  example,  the  ferric  hydroxide 
may  amount,  in  the  dark  grades,  to  25%  of  the  entire 
mass,  whilst  in  the  lighter  kinds  it  may  be  as  low  as 
3%.  It  is  very  rare  that  ochre  is  put  on  the  market 
in  its  native  condition,  being  mostly  subjected  to 
chemical  treatment  enabling  a  definite  shade  of  colour 
to  be  obtained.  This  will  be  gone  into  more  fully 

Yellow  Earth 

Yellow  earth  is  found  in  many  places  as  compact 
masses,  and  less  frequently  as  schistous  deposits.  It 
has  a  fine  earthy  fracture,  and  is  mostly  devoid  of 
lustre,  except  for  a  faint  shimmer  on  the  surface  of 
fracture ;  slightly  greasy  feel ;  and  a  tendency  to 
crumble,  in  water,  to  a  non-plastic  powder.  It  con- 
tains silica,  ferric  oxide  and  water  in  varying  pro- 
portions, and  the  yellow  earths  from  different  deposits 
always  vary  slightly  in  percentage  composition.  These 
differences  are  clearly  shown  in  the  following  analyses 


of    two     varieties     from    the     vicinity    of    Amberg 

(Bavaria)  : — 

i.  ii. 

Silica  .  .  33-23%         35-io% 

Alumina      .  .  14-21  I4'4° 

Ferric  oxide 


3776  36-80 

13-24  13-60 

When  heated,  the  colour  changes  gradually  to  red, 
and  the  earth  becomes  extremely  hard.  There  are 
several  recognised  commercial  grades,  the  price  of 
which  varies  mainly  in  accordance  with  the  colour 
and  fineness.  The  Amberg  variety  is  specially 
esteemed,  the  Hungarian  and  Moravian  kinds  being 
less  valuable. 

The  colour  not  being  particularly  good,  this  earth 
is  never  used  for  fine  work,  but  is  largely  employed  as 
a  yellow  wash  for  houses  and  as  ordinary,  distemper. 
It  may  also  be  used  as  an  oil  paint. 

Red  Ochre  is  a  less  important,  cheap  variety  of 
ochre,  chiefly  used  in  cheap  paints  and  for  low-priced 
wall-papers.  It  occurs  in  the  deposits  as  clayey 

Terra  di  Siena 

Terra  di  Siena  is  a  very  pure  form  of  ferric  hydroxide. 
When  ground,  the  light  to  dark  brown  lumps  furnish 
a  pale  to  dark  yellow  powder,  which  can  be  transformed 
into  a  number  of  gradations  by  burning.  In  spite  of 
its  handsome  colour,  this  pigment  is  deficient  in  cover- 
ing power,  in  addition  to  which  it  darkens  when  mixed 
with  varnish,  and  dries  slowly. 


Apart  from  the  small  quantities  of  native  vermilion 
handsome  enough  for  direct  use  as  painters'  colours- 


when  reduced  to  powder,  the  red  earths,  with  practically 
no  exception,  consist  of  ferruginous  minerals,  and  it 
is  only  within  a  recent  period  that  red  painters'  colours 
have  been  prepared  from  certain  chemical  waste 
products  from  manufacturing  processes.  In  all  cases, 
however,  compounds  of  iron  and  oxygen  constitute 
the  bulk  of  the  red  earths.  In  addition  to  ferric  oxide, 
which  is  the  chief  material  used  for  making  the  im- 
portant red  colours,  are  compounds  of  ferric  oxide  and 
water,  i.  e.  ferric  hydroxides.  The  ferric  oxide  pig- 
ments are  among  the  most  important  in  the  entire 
series  of  earth  colours,  being  on  the  one  hand  very 
cheap,  and  on  the  other  so  handsome  in  colour  that 
ferric  oxide  can  be  used  for  the  finest  paintings. 

Ferric  oxide  can  also  be  shaded  very  extensively  by 
a  fairly  simple  treatment,  so  as  to  furnish  a  whole 
range  of  very  handsome  shades. 

In  nature,  ferric  oxide  occurs  in  numerous  varieties 
of  one  and  the  same  mineral,  red  iron  ore,  which  is 
also  known  as  hematite,  blood  stone,  raddle,  etc. 

Red  Ironstone 

Red  hematite  occurs  native  as  rhombohedral  crystals, 
which  mostly  consist  solely  of  ferric  oxide,  and  may  be 
considered  as  pure  oxide  for  the  purposes  of  the  colour - 
maker.  The  difference  between  the  several  varieties 
is  due,  not  to  any  chemical  variation,  but  entirely  to 
changes  in  physical  structure.  The  varieties  with  a 
radial,  fibrous  structure  are  known  as  red  hematite, 
the  colour  of  which  ranges  from  blood  red  to  dark 
brown  and  is  frequently  accompanied  by  metallic 
lustre.  The  scaly  modification  of  this  mineral  forms 
micaceous  iron  ore,  and  is  usually  a  deep  iron  black. 


In  the  neighbourhood  of  volcanoes  it  is  frequently 
found  as  particularly  handsome  crystals. 

Iron  cream  (frosty  hematite)  is  the  name  given  to 
a  beautiful  cherry  red  variety,  which  easily  rubs  off, 
has  a  greasy  feel  and  is  composed  of  extremely  fine 

The  so-called  raddle  occurs  in  Nature  as  a  readily 
pulverulent  earthy  mass  of  ferric  oxide  contaminated 
more  or  less  with  extraneous  substances.  On  account 
of  its  abundance  and  low  market  price,  it  is  largely 
used  in  painting. 

Although  mixed  with  numerous  foreign  substances, 
certain  clay  ironstones,  oolitic  ironstones  and  siliceous 
ironstones  may  be  regarded  as  ferric  oxide  in  the 
sense  understood  by  the  colour -maker,  all  these  minerals 
having  a  deep  red  to  deep  brown  colour  and  being 
capable  of  rinding  advantageous  employment  as 

Ferric  oxide  is  distinguished  by  two  properties 
which  render  it  specially  valuable  to  the  colour -maker. 
When  combined  with  water,  its  colour  is  no  longer 
red,  but  a  handsome  brown ;  and,  on  the  other  hand, 
when  heated,  the  colour  passes  through  brown  into 
a  permanent  dark  violet.  By  suitable  treatment  of 
such  minerals  as  consist  mainly  of  ferric  hydroxide, 
mixtures  can  be  obtained  which  contain  the  oxide  and 
hydroxide  in  variable  proportions  and  give  a  whole 
range  of  shades  between  brown  and  red. 

The  preparation  of  these  colours  is  easy  when  very 
pure  red  ironstone  is  available.  The  somewhat  ex- 
pensive pigment,  Indian  red,  is — when  pure — really 
nothing  but  a  very  pure  ferric  oxide  of  Indian  origin. 
Ferric  oxide,  however,  often  contains  impurities  which 
considerably  influence  the  colour  of  the  product. 


Owing  to  the  fact  that  large  quantities  of  ferric  oxide 
are  formed  as  by-products  in  certain  chemical  processes 
which  are  carried  out  on  a  very  extensive  scale,  this 
oxide,  which  is  very  pure,  can  be  advantageously  used 
for  making  iron  pigments,  especially  as  its  application 
for  other  purposes  is  very  restricted,  and  it  can  there- 
fore be  had  at  a  very  low  price. 

The  following  analyses  show  the  composition  of 
a  number  of  red  ironstones,  Nos.  i,  2  and  3  being 
hematite  from  Froment,  or  Wetzlar,  No  4  from  Wetzlar, 
Nos.  5  and  6  hematite  from  Whitehaven,  No.  7  from 
Thuringia,  No.  8  from  Bohemia,  No.  9  from  Spain, 
No.  10  from  N.  America,  and  No.  n  from  England. 






lime  and 








2  -OO 

























-  — 



















T  ^           Man- 
Ir0n-      ganese. 


3Z.  "-• 





Loss  on 


33-64     o-io 

7-58     8-10 



Trace    0-19 


9     31-38     0-19 

0-06  29-95 



—       0-09 


10      62-54      1-93 

1-71       — 






ii      62-91    Trace    1-39     0-70 



0-05     o-n 


1            1 

There  are  certain  other  minerals  closely  allied,  both 
chemically  and  mineralogically,  to  red  ironstone, 
namely,  the  brown  hematites  or  ironstones  used  in  the 


manufacture  of  iron.  Brown  hematite  consists  of 
ferric  hydroxide,  Fe2O3H2O,  and  occurs  in  a  variety 
of  forms  in  Nature,  the  most  frequent  being  pea 
(oolitic)  ore,  which  owes  its  name  to  the  spherical 
shape  of  the  grains.  Some  brown  hematites  are 
decomposition  products  of  other  minerals,  and  contain 
sulphur  and  phosphorus  in  addition  to  ferric  hydroxide. 
Like  the  pure  hydroxide,  they  are  biown  in  colour, 
but  differ  therefrom  considerably  in  their  chemical 
behaviour  when  heated.  This  is  particularly  the  case 
with  the  so-called  bog  ore,  which  is  mostly  found,  as 
spongy  yellow-brown  to  black  masses,  in  swamps,  and 
owes  its  origin  to  the  decomposition  of  various  ferrugi- 
nous minerals.  It  varies  greatly  in  chemical  composi- 
tion and  occasionally  contains  up  to  about  50%  of  sand. 
The  amount  of  ferric  oxide  in  bog  ore  varies  between 
20  and  60%,  and  it  also  contains  7-30%  of  water, 
up  to  4%  of  P2O5,  small  quantities  of  ferrous  oxide 
and  manganese  hydroxide,  together  with,  in  most  cases, 
mechanically  admixed  organic  residues. 

The  phosphorus  content  makes  bog  iron  a  very 
inferior  material  for  smelting,  the  resulting  iron  being 
of  low  quality.  Nevertheless,  it  can  sometimes  be 
advantageously  used  in  making  earth  colours,  though 
the  products  cannot  lay  much  claim  to  beauty  of 


The  native  earth  pigments  known  by  this  name 
form  masses  of  the  colour  of  leather  to  dark  brown, 
with  a  conchoidal  fracture  and  an  earthy  appearance. 
Bole  chiefly  consists  of  iron  silicate  combined  with 
water,  some  varieties  containing  small  quantities  of 
alumina.  The  composition  fluctuates  very  considerably, 


most  varieties  containing  41-42%  of  silica,  20-25% 
of  alumina,  and  24-25%  of  water,  the  remainder 
consisting  of  ferric  oxide.  Some  kinds,  such  as 
Oravicza  and  Sinope  bole,  contain  only  31-32%  of 
silica  and  17-21%  of  water. 

Bole  is  used  as  a  paint  for  walls,  clapboards,  etc., 
and  is  only  mentioned  here  because  of  its  relationship 
to  the  ferric  oxide  pigments. 

Alum  Sludge 

Large  quantities  of  clarification  sludge  are  produced, 
in  alum  works,  as  the  sediment  from  the  red  liquors. 
This  sludge  consists  mainly  of  ferric  oxide,  with  small 
quantities  of  other  oxides  and  sulphuric  acid  (basic 
ferric  sulphate,  and  would  be  an  entirely  worthless 
by-product  except  for  the  fact  that  it  can  be  manu- 
factured into  pigments,  some  of  them  of  great  beauty. 

All  alum  makers  should  treat  this  residue  and  con- 
vert it  into  pigments,  which  they  could  put  on  the 
market  at  a  low  rate,  the  cost  of  preparation  being 
small.  Since  the  material  is  chiefly  composed  of  ferric 
oxide,  the  resulting  colours  are  very  similar  to  those 
obtained  from  iron  ores ;  and  all  shades,  from  yellow- 
brown,  through  red,  to  the  darkest  brown,  are 

Mine  Sludge 

The  water  frequently  present  in  iron  mines  some- 
times contains  large  quantities  of  sediment,  which 
consist  mainly  of  iron  ochre  and  can  be  advantageously 
worked  up  into  pigments.  There  is  scarcely  any 
need  to  mention  that  all  substances  containing  ferric 
oxide  can  be  used  for  making  any  of  the  pigments 
obtainable  from  the  oxide  itself,  the  only  difference 


between  the  various  raw  materials  being  their  degree 
of  purity,  so  that  it  is  not  always  so  easy  to  obtain  a 
certain  desired  shade  from  a  given  material  in  such 
beauty  as  is  furnished  by  another  material,  the  small 
quantities  of  impurities  associated  with  the  ferric 
oxide  having,  in  many  instances,  an  important  influence 
on  the  colour. 


Only  two  minerals  are  known  which  are  capable 
of  direct  use  as  blue  pigments,  viz.  vivianite  (native 
Prussian  blue)  and  copper  carbonate  (azurite,  ultra- 
marine), and  as  neither  of  them  is  particularly  hand- 
some, they  are  only  used  for  unimportant  work.  Lapis 
lazuli  is  no  longer  employed. 

Azurite,  or  Ultramarine 

This  mineral,  which  is  of  frequent  occurrence  with 
malachite  and  other  cupriferous  minerals,  forms  small 
crystals  of  a  beautiful  deep  azure  blue  consisting  of 
cupric  oxide  in  combination  with  carbon  dioxide  and 
water,  expressed  by  the  formula  2CuCO8,Cu(OH)2, 
or  Cu3(OH)2(CO3)2,  and  containing  69-19%  of  cupric 
oxide,  25-58%  of  CO2  and  5-23%  of  water.  The 
colour  of  the  powdered  mineral  is  much  paler  than 
that  of  the  crystals.  The  pigment,  which  is  used  for 
cheap  paints,  is  not  particularly  stable,  and  loses  much 
of  its  beauty  when  applied  to  plaster. 


This  mineral  occurs  in  many  places  as  crystalline 
masses,  but  also  forms  earthy  deposits,  some  of  which, 
especially  in  certain  bogs,  attain  considerable  thickness. 


The  colour  is  between  indigo  and  blackish  blue ; 
and  the  freshly  won  mineral  often  has  an  unsightly 
whitish  appearance,  which,  however,  soon  changes 
into  the  pure  blue.  The  cause  of  this  peculiarity  is 
due  to  the  fact  that  vivianite  originally  consisted  of 
hydrate d  ferrous  phosphate,  which  is  white,  this  com- 
pound being  transformed,  under  the  influence  of  the 
air,  into  the  blue  ferric  phosphate. 

Vivianite  contains  ferric  oxide,  phosphoric  acid  and 
water,  but  in  variable  proportions.  The  original 
composition,  expressed  by  Fe2(PO4)2  +  8H2O2,  corre- 
sponds to  43-03%  of  ferrous  oxide,  28-29%  of  P2O5 
and  28-68%  of  water;  but,  in  the  air,  part  of  the 
ferrous  phosphate  is  oxidised  to  basic  ferric  phosphate, 
so  that  the  content  of  ferrous  oxide  may  range  from 
9-75  to  42-71%,  and  that  of  ferric  oxide  between  1-12 
and  38-20%.  Vivianite  is  also  sold  as  blue  ochre,  and 
is  now  seldom  used  as  a  painters'  colour,  owing  to  the 
introduction  of  a  large  number  cf  artificially  prepared 
blues,  which  are  superior  to  vivianite  in  colour  and  are 
cheaply  made.  However,  it  can  still  find  application 
in  localities  where  it  is  obtainable  in  quantity. 


The  green  earth  pigments  comprise  green  earth 
(Verona  green)  and  malachite.  Like  the  blue  earths, 
they  cannot  lay  any  particular  claim  to  beauty,  but 
they  are  very  cheap,  and  consequently  are  largely 
used  where  low  price  is  the  chief  consideration. 

Green  Earth 

In  Nature,  green  occurs  as  an  entirely  non-crystalline 
earthy  mass,  which  is  probably  a  decomposition 


product  of  augite.  It  has  a  close,  earthy  fracture, 
a  colour  between  seladon  and  olive  green,  and  a  slightly 
greasy  appearance.  In  point  of  chemical  composition 
it  consists  of  silica,  alumina,  magnesia,  sodium, 
potassium,  ferrous  oxide  and  water,  the  usual  repre- 
sentative formula  being  ROSiO2  H2O,  in  which  RO 
symbolises  a  metallic  oxide. 

The  colour  is  due  to  ferrous  oxide  ;  and  if  left  exposed 
to  the  air  for  a  long  time,  or  subjected  to  powerful 
calcination,  the  great  affinity  of  ferrous  oxide  for 
oxygen  causes  the  colour  to  turn  red  and  red-brown. 

Green  earth  is  found  in  many  localities,  e.  g.  Bohemia, 
Hungary,  the  Tyrol  and  Cyprus,  the  finest,  however, 
occurring  near  Verona,  on  which  account  it  is  known  as 
Veronese  earth. 


The  commercial  pigment  consists  of  powdered 
malachite,  a  mineral  which  usually  occurs  in  compact 
masses  of  a  handsome  emerald  green  colour,  though 
isolated  lumps  exhibit  considerable  variation  in  shade, 
some  of  them  being  dark  green  and  others  very  pale. 
In  chemical  composition,  malachite  is  closely  allied  to 
azurite,  consisting  of  cupric  oxide,  carbon  dioxide 
and  water,  and  the  difference  is  entirely  one  of  per- 
centage proportions.  The  formula  is  CuCO3,  Cu(OH)2, 
or  Cu2(OH)2CO3,  corresponding  to  71-90%  of  cupric 
oxide,  19-94%  of  carbon  dioxide  and  8-16%  of  water. 

Powdered  malachite  (even  the  dark  green  varieties) 
is  always  rather  light  in  colour,  and  for  this  reason  is 
not  much  used.  Furthermore,  the  mineral  is  rather 
hard  (3-5),  and  is  consequently  difficult  to  grind;  in 
addition  to  which  the  mineral  is  fairly  expensive,  on 
account  of  its  employment  as  a  source  of  copper, 


particularly  fine  pieces  being  also  used  as  ornaments 
or  for  making  works  of  art.  Moreover,  like  all  copper 
compounds,  it  is  very  sensitive  to  the  action  of  sul- 
phuretted hydrogen,  and  liable  to  discoloration  in 
course  of  time. 


Numerous  minerals  are  adapted  for  the  manufacture 
of  brown  pigments.  On  the  basis  of  chemical  com- 
position, they  may  be  classed  in  two  groups;  those 
consisting  of  ferric  hydroxide,  and  those  in  which  the 
brown  colour  is  due  to  organic  substances. 

The  first  group  comprises  the  minerals  which  have 
already  been  mentioned  in  connection  with  the  red 
earth  pigments,  bole  and  brown  ochre  (umber),  Terra 
di  Siena,  Cologne  earth  and  a  number  of  other  earths 
rich  in  ferric  hydroxide  belonging  to  this  category.  The 
second,  or  organic  group,  includes  compounds  that 
are  very  rich  in  carbon  and  are  therefore  of  a  very 
dark  colour,  the  shades  ranging  from  light  brown  to 
black,  e.  g.  the  true  umbers  and  asphaltum. 


As  already  mentioned,  the  term  "  umber "  was 
formerly  applied  to  brown  varieties  of  ochre,  whereas 
at  present  it  is  extended  to  certain  masses  of  brown- 
coal  character,  often  interspersed  with  iron  ochre  and 
sometimes  containing  manganese.  Umber  generally 
consists  of  fairly  dense,  earthy  masses,  which  are 
dried  and  ground — after  crushing  and  levigation,  if 

Valuable  varieties  are  Cappagh  brown  and  Cale- 
donian brown,  both  with  a  reddish  tinge. 


It  is  thus  evident  that  "  umber  "  now  implies  two 
different  kinds  of  materials,  organic  masses  and  iron- 
manganese  compounds, 'which  can  also  be  used  as  oil 
paints.  These  umbers  can  also  be  extensively  shaded 
by  burning,  the  final  colour  being  particularly  influenced 
by  the  amount  of  manganese  compounds  present. 

The  carbonaceous  umbers  (Cassel  brown,  Carbon 
brown)  are  combustible,  and  mostly  leave  behind  a 
merely  small  residue  of  ash.  An  important  property 
of  these  umbers  is  their  partial  solubility  in  alkalis,  a 
peculiarity  which  is  utilised  for  the  preparation  of 
brown  wood  stains. 

A  sphaltum 

Asphaltum  forms  very  friable  dark  brown  to  black 
masses,  which,  in  contact  with  a  light,  easily  ignite 
and  burn  with  a  bright,  but  very  smoky,  flame,  dis- 
engaging a  peculiar,  "  bituminous  "  smell,  and  leaving 
only  a  very  small  quantity  of  ash. 

Extensive  deposits  of  asphaltum  are  found  at  the 
Dead  Sea,  the  Pitch  Lake  on  the  island  of  Trinidad, 
in  Dalmatia,  and  many  other  places,  where,  however, 
it  is  in  an  impure  condition  and  frequently  contains 
large  quantities  of  sand.  In  many  localities  the  rock 
is  impregnated  with  asphaltum,  which  makes  it  dark 
brown  to  black  in  colour  and  gives  rise  to  a  bitu- 
minous odour  when  rubbed. 

Peat  beds  sometimes  contain  pockets  of  a  mass  with 
a  handsome  brown  colour  and  consisting  of  a  mixture 
of  humic  acids  and  other  organic  substances  which 
may  be  ranked  with  the  humin  bodies  that  are  always 
formed  when  organic  matter  decomposes  in  presence 
of  an  insufficient  supply  of  oxygen.  These  bodies 
are  dark  coloured,  mostly  deep  brown,  rich  in  carbon, 


and,  to  some  extent,  similar  to  brown  coal  or  peat  in 
chemical  composition. 

Their  high  carbon  content  renders  these  substances 
very  inert  towards  chemical  reagents,  and  therefore 
particularly  adapted  for  the  preparation  of  painters' 
colours.  Genuine  Vandyke  brown,  which  is  the 
handsomest  brown  known,  is  an  earth  rich  in  humin 
compounds;  and  Cassel  brown  also  belongs  to  this 


The  colour  of  these  earths  is  entirely  due  to  carbon, 
and  pure  carbon,  a  certain  form  of  which  occurs  native, 
is  itself  used  as  a  pigment.  Actually,  there  are  only 
two  minerals  that  require  to  be  mentioned  in  this 
connection  :  black  schist  and  graphite. 

Black  Schist 

In  most  cases  this  is  a  clay  shale,  so  rich  in  carbon 
as  to  appear  deep  black.  In  commerce,  this  mineral 
is  also  erroneously  called  "black  chalk";  but  at 
present  it  is  seldom  used  as  a  pigment  or  drawing- 
material,  black  chalks  being  produced  far  more  cheaply 
than  the  expense  of  preparing  the  natural  article. 

Grey  clay  shales  are  used  for  making  grey  earth 
pigments  (stone  grey,  and  mineral  grey). 


This  mineral  is  found,  in  a  very  pure  state,  in  many 
localities,  celebrated  deposits  occurring  in  England, 
Siberia,  Bohemia  and  Bavaria,  whilst  North  American 
graphite  has  lately  come  into  prominence. 

Graphite  is  a  modification  of  pure  carbon,  and  is 


met  with  in  the  form  of  hexagonal  (rhombohedral) 
crystals,  usually  occurring  as  hexagonal  plates  with  a 
lustrous,  iron-black  colour.  It  rubs  off  easily,  and 
readily  burns  away,  leaving  a  very  small  amount  of 
ash,  when  subjected  to  a  very  high  temperature  in 
presence  of  air. 

The  principal  uses  of  graphite  are  as  an  anticorrosive 
paint  for  iron,  and  for  making  lead  pencils. 

As  already  mentioned,  the  term  "  earth  colours  " 
has  been  considerably  broadened  of  late.  Whereas, 
formerly,  it  was  restricted  to  colours  prepared  ex- 
clusively from  minerals  by  a  simple  treatment,  limited 
to  crushing,  levigation  or  calcination,  it  now  includes 
the  pigments  obtainable  from  large  by-products  of 
certain  chemical  processes.  This  latter  class  is 
especially  important  as  affording  an  opportunity  of 
utilising  products  formerly  considered  worthless  and 
whose  removal  often  entailed  heavy  expense. 

By  drawing  on  these  materials  the  industry  of  the 
earth  colours  has  greatly  enlarged  its  scope.  At 
present,  many  colours  of  this  kind  are  on  the  market, 
and  it  is  to  the  interest  of  many  manufacturers  to 
endeavour  to  utilise  certain  waste  products  in  the 
same  direction.  The  advantage  of  such  a  course 
hardly  needs  emphasising ;  but,  to  give  only  a  single 
example,  it  may  be  mentioned  that  the  manufacture 
of  fuming  sulphuric  acid  from  green  vitriol,  by  the  old 
process,  produces  residues  which  were  formerly  looked 
upon  as  quite  worthless,  and  sold  at  .very  low  prices, 
but  are  now  worked  up,  in  a  number  of  factories,  into 
very  handsome  and  durable  pigments. 



THE  preparation  of  the  raw  materials  for  the  purpose 
of  making  earth  colours  is  a  very  important  matter, 
because  many  minerals  or  pigmentary  earths  merely 
require  mechanical  treatment  to  render  them  at  once 
fit  for  use.  The  mechanical  preparation  differs  con- 
siderably, in  accordance  with  the  raw  material  under 
treatment,  substances  that  are  found  native  in  a  finely 
powdered  condition  only  needing,  for  the  most  part, 
to  be  levigated. 

It  rarely  happens,  however,  that  the  raw  material 
occurs  in  condition  for  use  direct,  an  example  of  this 
kind  being  afforded  by  the  finest  clays  or  ochres. 
Whilst  these  are  found  in  a  state  of  extremely  fine 
powder,  they  nearly  always  contain  certain  quantities 
of  sandy  ingredients  or  even  large  lumps  of  foreign 
minerals,  and  therefore  require  levigating.  Sometimes 
they  need  crushing  as  well,  the  small  particles  cohering 
so  strongly  that  mere  treatment  with  water  (levigation) 
is  unable  to  separate  them.  Mechanical  force  is  there- 
fore necessary,  a  passage  through  grooved  rollers  being 
generally  sufficient  to  crush  the  lumps;  but  in  some 
cases  stamps  have  to  be  used. 

When  solid  materials  have  to  be  treated,  mechanical 
appliances  must  always  be  used,  their  selection  depend- 
ing on  the  materials  in  question.  Thus,  gypsum,  for 



example,  can  be  crushed  with  ordinary  rolls  or  mill 
stones,  its  degree  of  hardness  being  so  very  low  (2) 
that  it  can  be  scratched  with  the  finger-nail. 

If,  however,  the  material  to  be  reduced  is  limestone, 
which  belongs  to  the  third  degree  of  the  scale  of  hard- 
ness (can  only  be  scratched  with  an  iron  nail),  or  heavy 
spar  (hardness  3-3*5),  very  powerful  stamps  or  edge- 
runners  must  be  employed  to  break  it  down  into  small 
lumps,  which  can  then  be  further  reduced,  without 
any  special  difficulty,  in  an  ordinary  mill. 

It  is  thus  evident  that  a  great  variety  of  mechanical 
appliances  are  used  in  the  manufacture  of  earth  colours. 
Before  going  into  their  construction  it  is  necessary  to 
point  out  that,  whatever  the  mechanical  treatment 
employed,  a  considerable  expenditure  of  mechanical 
force  is  entailed ;  and  more  power  is  needed  when 
mixtures  have  to  be  prepared.  It  is  therefore  essential, 
in  planning  a  factory  for  making  earth  colours  on  a 
large  scale,  to  make  provision  for  ample  motive  power. 

This  power  may  be  supplied  by  a  steam  engine ; 
but  it  must  not  be  forgotten  that  the  prime  cost  and 
running  expenses  of  such  an  engine  are  considerable, 
and  form  an  important  item  in  view  of  the  low  value 
of  most  earth  colours.  Consequently,  it  is  highly 
important  to  be  able  to  generate  motive  power  as 
cheaply  as  possible. 

Now,  the  cheapest  and  most  uniform  source  of  power 
is  water;  and  therefore,  wherever  the  conditions  allow 
of  the  erection  of  the  colour  works  near  a  stream  or 
river,  which  can  supply  the  power  to  run  the  various 
machinery,  the  most  favourable  circumstances  will 
have  been  secured,  the  power  being  obtained  at  mini- 
mum cost,  whilst  the  upkeep  of  the  motor  cannot  be 
very  great.  If  there  is  sufficient  head  for  the  water 


to  be  run  through  a  trough  over  the  top  of  the  leviga- 
tion  tanks,  the  conditions  will  be  ideally  favourable. 

Wind  power  costs  nothing,  once  the  motor  has  been 
installed;  but  unfortunately,  one  is  dependent  on 
the  weather,  and  sometimes  there  is  not  enough  wind, 
for  days  together,  to  drive  the  sails  at  all,  and  therefore 
all  the  operations  have  to  be  stopped,  including  leviga- 
tion,  the  water  for  which  has  to  be  raised  by  a  windmill 

In  districts  where  the  winters  are  severe,  water 
power  may  also  fail  and  work  have  to  be  stopped; 
and  consequently,  even  when  water  power  is  the  prime 
source  of  energy,  a  steam  engine  must  be  installed  as 
a  stand-by,  being,  of  course,  only  used  when  the  main 
source  of  power  gives  out  or  proves  insufficient. 

The  machines  employed  for  preparing  the  raw 
materials  in  the  manufacture  of  earth  colours  may  be 
divided  into  the  following  groups  :— 

Machines  operating  entirely  by  pressure  :  crushers ; 
machines  acting  by  impact  :  stamps ;  those  acting 
by  impact  and  pressure  :  vertical  mills  (edge -runners), 
ball  mills,  centrifugal  mills;  and,  finally,  machines 
with  a  frictional  action  :  grinding  mills.  Then  there 
are  the  levigating  machines,  which  do  not  reduce  the 
material  but  separate  the  coarser  particles  from  the 
finer.  The  construction  of  the  foregoing  machines 
is  a  matter  for  the  machinery  manufacturer  rather 
than  the  maker  of  earth  colours ;  but  as  the  business 
of  the  latter  is  dependent  on  them,  a  short  description 
is  considered  necessary.  The  selection  depends,  on 
the  one  hand,  on  the  nature  of  the  materials  to  be 
treated,  and,  on  the  other,  on  the  size  of  the  works, 
since  a  manufacturer  who  has  to  deal  with  large  quan- 
tities of  a  given  raw  material  will  require  different 


machines  from  those  used  on  a  small  scale.  The  sole 
purpose  of  the  following  description  is  to  indicate  to  the 
colour  maker  the  way  in  which  the  reduction  of  the 
raw  material  can  be  accomplished. 


Crushers  and  Breakers. — Crushers  usually  consist 
of  grooved  iron  rollers  revolving  on  horizontal  axes. 
One  of  the  rollers  is  fixed,  the  other  being  adjustable 
by  screws,  in  order  that  lumps  of  different  sizes  may 
be  treated  in  one  and  the  same  machine,  which  may 
be  employed  either  to  turn  out  a  roughly  crushed 
product,  or  to  reduce  it  to  a  certain  degree  of  fineness. 

If  several  pairs  of  crushing  rollers  be  mounted  in 
series,  and  each  set  a  little  closer  than  its  predecessor, 
the  material  can  be  reduced  progressively  from  large 
lumps  to  a  fairly  fine  powder. 

Each  pair  of  rollers  is  geared  together  by  pinions, 
and  is  turned  in  such  a  way  as  to  draw  the  material 
in  between.  If  the  gear  pinions  have  the  same  number 
of  teeth,  the  two  rollers  will  revolve  at  the  same  speed 
and  will  then  merely  crush  the  material  into  lumps 
of  a  size  depending  on  the  distance  at  which  the  rollers 
are  set  apart. 

Nevertheless,  by  simply  altering  the  gear  ratio  of 
the  pinions,  the  crushing  action  of  the  rollers  can  be 
supplemented  by  a  grinding  action,  a  much  finer  powder 
being  then  obtainable  than  otherwise,  the  one  Droller 
running  at  a  higher  speed  than  the  other. 

These  crushers  differ  in  strength  of  construction, 
very  strongly  built  machines  being  required  for  dealing 
with  large  lumps  of  hard  material,  whereas  substances 
of  low  crushing  strength,  such  as  clay  or  other  earthy 



materials,  can  be  treated  in  much  lighter  machines. 
In  any  case,  however,  it  is  advisable  to  have  the  machine 
stronger  than  is  absolutely  necessary  for  the  work  in 
view;  for,  although  the  prime  cost  is  thus  increased, 
the  outlay  on  repairs  will  be  reduced,  and  the  machines 
can,  if  necessary,  be  used  on  harder  material  as  well. 
The  framework  supporting  the  rollers  should  always 
consist  of  a  strong  iron  casting;  and  the  machine 

FIG.  i. 

should  be  set  up  as  close  as  possible  to  the  engine  or 
motor,  to  minimise  the  loss  of  power  in  transmission 
through  long  shafting,  etc. 

Fig.  i  represents  a  breaker  (made  by  the  Badische 
Maschinenfabrik,  Durlach),  suitable  for  the  rough 
crushing  of  clayey  materials  supplied  in  large  lumps. 
It  can,  however,  also  crush  shale,  lime,  chalk,  as  well 
as  hard,  sticky  masses  which  would  clog  up  a  stone- 

The  material  fed  into  this  breaker  is  gripped  at  once 


by  the  poweiful  projecting  teeth,  which  are  connected 
together  by  sharp-edged  ridges,  and  is  crushed  in  such 
a  way  that  it  can  be  easily  reduced  still  further  by  a 
succeeding  pair  of  smooth  rollers. 

The  granulator  (Fig.  2),  made  by  the  same  firm,  is 
an  example  of  a  machine  for  crushing  harder  materials. 

FIG.  2. 

It  is  similar  in  construction  to  a  stone-breaker,  but 
differs  in  the  movement  of  the  jaws,  and  combines 
the  properties  of  breaker  and  grinder,  inasmuch  as 
it  tears  the  material  as  well  as  crushes  it.  The  figure 
shows  the  machine  adapted  for  direct  electric  drive. 
If  necessary,  these  granulators  can  be  fitted  with 
classifying  jig  screens. 

Stamps. — Stamps  or  stamping-mills  have  been  used 


from  prehistoric  times,  and  were  probably  employed 
for  reducing  hard  materials  long  before  the  introduction 
of  grinding-mills.  The  underlying  principle  of  the 
stamping -mill  is  very  simple.  The  material  to  be 
reduced  is  placed  in  a  trough  or  mortar,  and  the  ram 
or  head,  which  is  of  considerable  weight,  is  raised  by  a 
mechanical  device  and  then  allowed  to  fall  freely,  from 
a  certain  height,  on  to  the  material  underneath,  which 
it  crushes.  The  heavier  the  head  and  the  greater 
the  height  of  fall,  the  greater  the  effect  produced.  As 
a  rule,  a  large  number  of  stamps  are  mounted  together, 
and  in  such  a  way  that  half  of  them  are  being  lifted 
while  the  other  half  are  falling.  Either  a  separate 
mortar  or  trough  is  arranged  under  each  stamp,  or 
else  the  whole  drop  into  a  common  trough  charged 
with  the  material  under  treatment.  Sometimes  a 
lateral  movement  is  imparted  to  the  material  in  the 
trough,  so  as  to  bring  it  under  the  action  of  all  the  stamps 
in  succession. 

Although  the  construction  of  stamping -mills  in 
general  appears  simple,  various  modifications  are 
employed  for  different  purposes. 

As  a  rule,  a  single  passage  through  a  stamping -mill 
is  not  sufficient  to  reduce  the  material  completely 
to  the  desired  fineness,  the  first  product  always  con- 
taining large  and  coarse  fragments  of  various  sizes, 
as  well  as  fine  powder. 

If  the  latter  were  left  in  with  the  larger  pieces  for 
the  second  stamping  it  would  impede  the  work,  and 
the  stamping-mill  should  therefore  be  provided  with 
means  for  classifying  the  material  discharged  from  the 
trough,  to  separate  the  fine  from  the  coarse  and  grade 
the  latter  into  sizes.  This  is  usually  effected  by  means 
of  a  grading-screen. 


Stamping -mills  are  chiefly  used  for  reducing  brittle 
materials.  A  number  of  stamps  arranged  in  a  row  are 
alternately  lifted,  by  means  of  cams  mounted  on  a 
common  shaft,  and  then  let  fall  on  to  the  material 
lying  on  a  solid  plate,  or  else  on  a  grating  through  which 

FIG.  3. 

the   crushings    fall.     Fig.   3  is   a    stamping-mill    con- 
structed by  H.  F.  Stollberg,  Offenbach. 

These  mills  are  very  strongly  built,  as  independent 
units,  the  frame  being  of  cast-iron  and  the  rams  of  best 
wrought -iron  with  interchangeable  chill-cast  heads. 
In  some  mills  the  stamps  are  rotated  during  the  up- 
stroke, in  order  to  equalise  the  wear  on  the  heads,  and 
also  to  economise  power. 



The  grating  or  trough  holding  the  material  is  per- 
forated with  holes,  the  diameter  of  which  varies  with 
the  material  under  treatment  and  the  desired  degree 
of  fineness  in  the  product.  To  increase  the  efficiency 
of  the  mill,  the  grating  or  trough  is  adapted  to  move 
while  the  mill  is  running,  in  order  to  clean  itself  auto- 

FIG.  4. 

matically.     These   mills   are   made   in   different   sizes, 
with  2,  4,  6,  or  8  heads. 

Edge-runners. — This  type  of  crusher  is  highly  suitable 
for  reducing  earth  colours  in  large  works.  The  special 
feature  of  the  type  is  that  both  stones  are  mounted 
vertically  and  turn  on  a  common  shaft  in  the  same  way 
that  a  cart  wheel  does  on  its  axle.  These  runners  are 
particularly  useful  for  reducing  clay,  chalk  and  other 
earth  colours,  which  have  to  be  dealt  with  in  large 


quantities.  They  will  also  crush  fairly  large  lumps,  and 
they  can  therefore  be  used  for  the  further  reduction  of 
materials  roughly  crushed  in  a  breaker,  etc.  The 
material  may  be  treated  in  either  the  wet  or  dry  state, 
only  slight  alteration  being  needed  to  change  from  one 
method  to  the  other. 

There  are  numerous  different  patterns  of  edge -runner, 

FIG.  5. 

but  all  of  them  can  be  divided  into  two  groups,  viz. : 
mills  with  stationary  troughs,  whilst  the  shaft  carrying 
the  runners  rotates;  and  those  in  which  the  trough 
revolves,  and  the  stones  merely  turn  on  the  stationary 
horizontal  shaft. 

Comparison  of  the  efficiency  of  the  two  types  has 
shown  that  the  revolving-trough  type  is  the  better, 
giving  a  larger  output  per  unit  time  with  a  reduced 
consumption  of  power.  Figs.  4  and  5  show  a  vertical 
section  and  plan  respectively  of  this  type  of  edge-runner. 
The  trough  G  is  turned  by  means  of  a  toothed  crown 


gearing  with  the  bevel  pinion  0  mounted  on  an  over- 
head shaft  C  driven  by  a  belt  pulley  N. 

The  bearings  of  the  vertical  shaft  /  of  the  trough 
are  situated  at  L  and  M.  The  runners  H  are  loosely 
mounted  on  the  fixed  horizontal  shaft  E  and  revolve 
in  consequence  of  the  friction  between  them  and  the 
material  in  the  trough.  As  the  latter  revolves,  the 
material  is  continuously  pushed  aside  by  the  runners, 
and  is  again  brought  under  them  by  the  action  of 

The  great  advantages  afforded  by  edge-runners, 
in  consequence  of  their  simplicity,  easy  management 
and  low  wear  in  comparison  with  other  grinding 
appliances,  have  led  to  their  reintroduction  on  a  large 
scale.  It  should,  however,  be  borne  in  mind  that  the 
edge-runner  mill  must  be  of  a  pattern  suitable  to  the 
materials  it  will  have  to  treat.  The  method  of  drive 
usually  depends  on  local  conditions.  The  revolving- 
trough  type  is  chiefly  useful  for  mixing,  on  account 
of  the  ease  with  which  the  materials  can  be  charged. 

The  capacity  of  edge-runner  mills  depends  on  the 
nature  of  the  material,  the  diameter  and  weight  of 
the  runners,  the  speed  at  which  they  are  run,  and  also 
on  the  rate  at  which  the  reduced  material  is  discharged 
in  order  to  give  place  to  fresh  portions  of  the  charge. 
This  is  effected  by  means  of  two  sets  of  scrapers,  the 
individual  members  of  which  can  be  adjusted  in  any 
direction.  Their  ploughing  action  also  greatly  assists 
the  mixing  effect. 

Fig.  6  illustrates  an  edge-runner  mill  with  revolving 
trough  and  overhead  drive;  and  Fig.  7  one  with 
stationary  trough  and  bottom  drive;  both  made  by 
the  Badische  Maschinenfabrik,  Durlach.  The  runners 
are  of  grey  cast-iron,  chill -castings  or  cast -steel  being 


used  for  crushing  hard  materials.  The  trough  in 
all  cases  is  lined  with  detachable  chill-cast  plates. 
Special  attention  is  bestowed  on  the  lubrication  of 
all  the  moving  parts,  and  all  the  lubricators  are  easily 

The  main  shafts  of  the  fixed-trough  machines  have 

FIG.  6. 

forged  cranks,  and  the  metal  crank  bearings  are  pro- 
vided with  dust  caps.  All  the  shaft  journals  run  in 
detachable  metal  bushes. 

A  special  advantage  attaching  to  this  type  is  the 
automatic  screening  device  and  the  returning  of  the 
screen  residue.  In  some  cases,  complicated  appliances 
are  employed  to  return  the  coarse  residue  from  the 
screen,  bucket  elevators,  worm  conveyors,  etc.,  all 


entailing  increased  motive  power,  not  inconsiderable 
wear,  and  a  higher  prime  cost;  but  in  this  instance 
the  object  is  achieved,  without  extra  power  or  wear, 
by  very  simple  means.  The  dust-proof  shell  enclosing 
the  runners  and  screen  is  provided  with  large  doors 
and  charging  hoppers. 

FIG.  7. 

The  motive  power  required  to  drive  edge-runner 
mills  depends  on  the  dimensions  of  the  mill  and  on  the 
class  of  material  to  be  treated;  the  larger  the  mill 
and  the  coarser  the  material,  the  more  power  needed 
to  drive  it. 

This  type  is  the  more  suitable  for  raw  materials 
that  are  of  an  earthy  character,  so  that  all  that  is 


FIG.  8. 



necessary  is  to  destroy  the  cohesion  of  the  particles, 
as  is  the  case,  for  example,  with  clay  and  all  earthy 

The  wet  method  of  crushing  with  edge  runners  is 

FIG.  9. 

particularly  suitable  as  a  preliminary  to  levigation.  A 
machine  arranged  for  this  purpose  is  shown  in  Fig.  8. 
It  consists  of  two  sets  of  edge  runners,  one  with  fixed, 
and  the  other  with  revolving  trough.  The  material 
is  introduced  by  hand,  or  by  suitable  charging  mechan- 


ism,  into  the  upper,  fixed-trough  machine,  where  it 
is  continuously  sprinkled  with  water  and  kneaded 
by  the  one  runner,  and  is  passed  thence  to  the  second 
roller  which  forces  it  through  the  slotted  bed  into  the 
bed  of  the  lower  set.  The  slotted  beds  of  the  upper  and 

FIG.  10. 

lower  set  are  offset ;  and  the  chief  function  of  the  lower 
set,  with  rotating  bed,  is  to  secure  intimate  admixture 
of  the  material  which,  in  most  cases,  is  already 
sufficiently  reduced. 

Ball  Mills. — Ball  mills  are  generally  used  for  crushing 
dry  materials  to  fine  powder.  The  mill  shown  in 
Fig.  9  is  a  typical  form  of  grinding  drum  enclosed  in 


a  dust-proof  casing,  the  latter  being  provided,  at  the 
top,  with  an  opening  connected  to  the  dust  exhaust 
pipe.  The  discharge  outlet  at  the  bottom  can  be 
closed  by  a  slide. 

The  drum  is  provided  with  two  strong  lateral  shields 
or  cheeks  (Fig.  10),  one  of  which  carries  the  inter- 
changeable cross-arm  and  the  charging  hopper.  Both 
cheeks  are  lined  with  detachable  chill-cast  plates, 
to  take  up  the  wear.  The  bed  is  formed  of  heavy 
steel  bars  (which  can  be  turned  round),  between  which 
are  arranged  adjustable  slits  for  the  discharge  of  the 
reduced  material.  Guard  sieves  are  mounted  all 
round,  and  close  to,  the  bed,  and  interchangeable  fine 
screens  surround  these  in  turn.  The  mesh  of  the  fine 
screens  determines  the  fineness  of  the  product,  and  the 
residue  falls  down  on  to  a  plate  which  returns  it  to  the 
interior  of  the  drum.  The  reduction  of  the  charge 
is  effected  by  a  number  of  very  hard,  forged  steel  balls 
of  various  sizes. 

The  mill  must  be  run  in  the  direction  marked  by  the 
arrow  on  the  outer  shell,  so  that  the  residue  on  the 
screens  can  be  returned  to  the  drum  by  the  plate  pro- 
vided for  that  purpose ;  and  the  prescribed  working 
speed  must  be  maintained.  The  mill  must  not  be 
overloaded.  The  impact  of  the  balls  should  be  mild, 
but  distinctly  audible.  Overloading  reduces  the  out- 
put. Idle  running  causes  the  most  wear,  since  the 
balls  then  roll  directly  on  the  bed,  which,  of  course, 
should  be  prevented  as  far  as  possible.  The  feed  is 
continuous;  and,  of  course,  only  dry  material  should 
be  introduced. 

When  the  balls  have  lost  size  and  weight  through 
wear,  they  must  be  replaced  by  a  fresh  set. 

Pulverisers. — Pulverisers  are  the  best  form  of  crusher 


for  tough  and  not  over-hard  materials.  They  are 
simple  and  strong  in  construction,  of  high  capacity 
with  comparatively  small  consumption  of  power, 
and  furnish  a  good,  uniform  product,  the  size  of 

FIG.  ii. 

which  ranges  from  fine  powder  to  coarse  granules, 
according  to  the  screens  used  and  the  class  of  material 

The   crushing   is   effected   by   a    cross-arm   beater, 
composed  of  four  to  six  radial  steel  arms  on.  a  divided, 


cast -steel  hub,  keyed  on  to  the  horizontal  shaft.  The 
arms  are  hardened,  and  are  adjustably  and  detach  ably 
mounted  on  the  hub. 

The  beating  action  of  the  arms,  which  run  at  high 
speed,  forces  the  material  against  the  studded  surface 
of  the  hardened  cheeks  of  the  machine  and  also  against 
the  hardened  square  steel  bars  forming  the  periphery, 
the  repeated  impact  of  the  material  on  itself,  as  well 
as  against  the  arms  and  bars,  progressively  reducing  it 
until  small  enough  to  fall  through  the  screen  on  the 
under  half  of  the  casing,  into  a  closed  receptacle  below. 
The  screen  mesh  varies  according  to  the  degree  of 
fineness  required. 

The  peripheral  bars  are  mounted  in  a  very  simple 
manner,  and  in  such  a  way  that  when  one  edge  of  the 
bars  is  worn,  a  quarter  turn  brings  a  fresh,  sharp  edge 
into  operation,  so  that  all  four  edges  of  each  bar  can 
be  utilised. 

To  prevent  the  escape  of  dust,  the  machine  is  pro- 
vided with  an  air-circulation  chamber,  which  maintains 
the  flow  of  air  in  continuous  circulation,  the  resulting 
strong  draught  also  drawing  the  fine  material  through 
the  screen  and  keeping  the  meshes  open.  By  this  means 
the  capacity  of  the  pulveriser  is  considerably  increased. 
The  interchange  of  the  crushing  organs  and  screens, 
and  also  the  cleaning  of  the  machine,  can  be  effected 
without  difficulty  or  loss  of  time. 

The  charge  is  introduced  through  a  feed  hopper  at  the 
side,  and  may  vary,  according  to  the  size  of  the  machine, 
from  nut  size  to  lumps  twice  as  large  as  a  man's  fist. 
If  necessary,  suitable  mechanical  feed  devices  can  be 

Disintegrators  (Figs.  12  and  13). — This  type  of  machine 
is  used  for  reducing  medium-hard  or  soft  materials, 


especially  where  it  is  desired  to  obtain  a  comparatively 
large  output  of  a  gritty  product. 

In  the  patterns  shown,  the  main  shaft  is  of  steel, 
with  dust-  and  dirt-proof  red-brass  bearings  with 
pad  or  ring  lubrication.  The  spindle  case  draws  out 
to  facilitate  cleaning.  Mechanical  feeding  attachments 
can  be  provided. 

FIG.  12. 

According  to  local  conditions,  the  disintegrator  can 
be  mounted  either  on  a  brick  foundation,  with  lateral 
discharge  outlet  into  a  storage  bin,  or  on  a  raised  grating 
of  iron  joists. 

If  the  product  is  to  be  conveyed  to  a  distance,  it 
is  advisable  to  have  a  hopper-shaped  collector  leading 
directly  to  a  worm  conveyor  or  bucket  elevator. 

The  arrangement  shown  in  Fig.  13,  in  which  the 
disintegrator  is  mounted  on  a  dust-proof  cast-iron 



collector,  has  been  found  very  suitable  for  colour  works 
of  various  kinds  (aniline,  lead,  mineral  and  other 
colours),  particularly  on  account  of  the  suppression 
of  dust ;  whilst  the  automatic  charging  worm  greatly 
increases  the  capacity  as  compared  with  charging  by 

FIG.  13. 

The  effect  of  levigation  is  based  on  the  circumstance 
that  bodies  of  greater  density  than  water  remain  longer 
in  suspension  in  that  medium  in  proportion  as  the 
fineness  of  their  particles  increases.  This  treatment 
consequently  enables  the  finer  portions  of  a  substance 


to  be  mechanically  separated  from  the  coarser.  Leviga- 
tion  is  extensively  practised  in  colour  works  because 
it  furnishes  powder  of  finer  grain  than  can  be  obtained 
by  even  the  most  careful  grinding. 

The  appliances  used  for  levigation  may  be  of  a  very 
simple  character,  consisting  only  of  several  tubs  or 
tanks,  mounted  in  such  a  way  that  the  liquid  con- 
tained in  one  can  be  run  off  into  the  one  next  below. 
With  this  primitive  plant,  the  material  to  be  levigated 
is  stirred  up  in  the  water  in  the  uppermost  tub  and 
left  to  settle  until  the  coarsest  particles  may  be  assumed 
to  have  settled  down,  whereupon  the  turbid  water  is 
drawn  off  into  another  tub,  in  which  it  is  left  to  settle 
completely.  When  the  clear  liquid  has  been  carefully 
drawn  off,  a  fine  sludge  is  left  in  the  bottom  of  the  tub, 
consisting  of  the  fine  particles  of  material  mixed  with 

When  a  particularly  fine  powder  is  required,  a  single 
levigation  does  not  always  suffice,  but  the  liquid  in 
the  second  tub  must  be  left  to  settle  for  a  short 
time  only,  and  then  run  into  a  third  for  complete 

A  well-designed  levigator  for  treating  large  quantities 
of  powder  is  illustrated  in  Fig.  14.  A  stirrer  R,  driven 
by  cone  gearing,  is  arranged  in  a  wooden  or  stone  vat 
G.  The  levigating  water  enters  close  to  the  bottom  of 
the  vat,  through  the  pipe  W.  When  G  is  half  full  of 
water,  the  stirrer  is  set  running,  and  the  substance  to 
be  levigated  is  added.  After  a  while,  the  water  laden 
with  the  levigated  powder  begins  to  run  off  at  A  into 
the  long  narrow  trough  7\  provided,  at  the  opposite 
end  from  A,  with  a  number  of  perforations  through 
which  the  water  runs  into  the  trough  TV  From  this 
it  escapes  through  the  perforations  into  the  trough  T3. 



and  thence  successively  into  T4  and  T5,  finally  dis- 
charging into  the  large  tank  S. 

The  coarsest  and  heaviest  of  the  water-borne  particles 
deposit  in  the  trough  7\,  finer  particles  settling  down 
in  T2,  and  so  on  in  succession,  until  the  water  reaching 
the  tank  S  contains  only  the  very  finest  of  all  in  sus- 

FIG.  14. 

pension,  these  taking  a  long  time  to  settle  down  to 
the  bottom.  The  deposit  in  the  upper  troughs  can 
be  returned  to  the  vat,  whilst  that  in  the  lower  ones 
will  be  fine  enough  to  dry  as  it  is.  The  residue  in 
the  vat  is  discharged  through  Z  when  the  operation 
is  finished. 

It  will  be  evident  that  the  fineness  of  the  product 
depends  on  the  number  and  length  of  the  troughs  T, 
the  larger  these  factors  the  more  delicate  will  be  the 


particles  remaining  in  prolonged  suspension  in  the 

Many  earth  colours  require  no  treatment  beyond 
levigation  to  fit  them  for  use  in  paints.  This  is  the  case 
with,  e.  g.,  the  white  clays ;  and  certain  grades  of  ferric 
oxide,  which  occur  native  in  the  state  of  fine  powder, 
may  also  be  included  in  this  category.  In  many  cases, 
however,  if  large  quantities  of  a  finely  pulverulent 
mineral  be  stirred  up  with  water  and  left  to  stand, 
the  deposited  solid  matter  forms  such  a  highly  coherent 
mass  that  it  can  only  be  distributed  in  water  with 
difficulty,  the  fine  particles  adhering  so  firmly  together 
that  it  is  hardly  possible  to  stir  them  up  again  completely 
in  the  liquid  by  means  of  a  paddle. 

Nevertheless,  this  can  be  easily  effected  by  using 
a  special  appliance  of  the  kind  employed  by  starch 
manufacturers  for  a  similar  purpose,  viz.  the  levigation 
of  starch.  This  apparatus  is  designed  in  such  a  way 
that  the  pulpy  charge  of  material  is  gradually  and  com- 
pletely disseminated  in  the  introduced  liquid. 

Fig.  15  shows  a  device  of  this  kind,  consisting  of  a 
circular  vessel  provided  with  a  step  bearing  for  a  ver- 
tical shaft  driven  by  cone  pinions.  The  lower  part 
of  the  shaft  is  provided  with  a  thread,  on  which  a  nut 
is  adapted  to  travel  up  and  down.  By  means  of  rods, 
this  nut  is  connected  to  a  wooden  cross-bar  provided 
with  stiff  bristles  on  its  lower  face.  A  horizontal  handle 
is  attached  to  the  nut.  The  water  is  admitted  through 
the  pipe  on  the  right. 

In  working  the  apparatus,  the  shaft  is  rotated  and 
the  handle  held  firmly,  thus  causing  the  nut  and 
attached  cross-bar  to  rise  to  the  limit  of  its  travel. 
The  levigating  liquid,  mixed  with  the  material  under 
treatment,  is  then  admitted,  until  the  vessel  is  full, 


and  when  the  solids  have  completely  subsided,  the 
clear  liquid  is  drawn  off,  and  the  operation  is  repeated 
until  a  thick  layer  of  sediment  has  accumulated  on 
the  bottom  of  the  vessel. 

To  levigate  this,  the  cross-arm  carrying  the  bristles 
is  lowered  until  it  just  touches  the  surface  of  the  deposit, 
and  a  continuous  stream  of  water  is  admitted  through 

FIG.  15. 

the  pipe  at  the  side.  The  bristles  gradually  disseminate 
the  upper  layers  of  the  sediment  in  the  water,  which 
becomes  turbid  and  is  then  drawn  off  into  another 
vessel,  cement-lined  pits  being  used  in  the  case  of 
large  quantities.  When- the  brushes  no  longer  encounter 
any  of  the  sludge,  the  cross-arm  is  lowered  sufficiently 
to  stir  up  another  layer ;  and  in  this  way,  large  quan- 
tities of  solid  matter  can  be  distributed  in  water.  If 
the  cross-arm  is  rotated  at  low  enough  speed,  the 


coarser  particles  of  material  keep  on  settling  down  again, 
and  the  collecting  vessels  will  receive  only  the  finest 

In  addition  to  the  mechanical  separation  of  coarse 
and  fine  particles,  levigation  accomplishes  another 
purpose,  namely  that  the  prolonged  contact  of  the 
treated  material  with  water  dissolves  out  any  admixed 
soluble  constituents  which  might  affect  the  quality 
of  the  colour,  the  latter  being  left  in  a  purified  condition. 

For  successful  levigation  it  is  essential  that  the  charge 
should  be  in  a  sufficiently  fine  condition  at  the  outset. 
Clayey  raw  materials  require  no  preliminary  treatment 
other,  perhaps,  than  passing  them  through  a  disinte- 
grator, whereas  hard,  crystalline  substances  must 
first  be  ground  in  a  wet  mill,  such  as  an  edge-runner 
mill  with  stationary  bed,  into  which  the  materials  are 
fed  with  an  admixture  of  water,  provision  being  made 
for  keeping  the  charge  under  the  runners  all  the 
time.  The  crushed  material  is  screened  previous  to 

In  the  levigation  process  a  few  vessels  of  large  size 
are  preferable  to  a  number  of  small  ones.  The  nature 
of  the  material  will  determine  whether  any  stirrers 
are  required  or  not,  these  being  unnecessary  in  the 
case  of  the  pigmentary  earths,  which  naturally  remain 
a  long  time  in  suspension  and  therefore  do  not  require 
stirring  up. 

The  pulpy  levigated  material  is  taken  out  of  the  tubs, 
etc.,  drained  (if  necessary)  and  dried.  The  draining 
may  be  effected  in  bags,  or — in  large  plants — filter 
presses  or  hydro-extractors.  In  these  latter  instances, 
pumps  will  be  provided  for  feeding  the  sludge  direct 
to  the  presses,  and  conveyors  for  delivering  the  pressed 
material  to  the  drying-plant. 



The  levigated  colour  earths  form  a  stiff  pulp  con- 
taining a  large  quantity  of  water,  which  can  be  elimi- 
nated in  various  ways.  Usually,  the  mass  is  dried  by 
spreading  it  out  thinly  on  boards  and  leaving  it  exposed 
to  the  air  until  it  has  become  solid ;  or  else  it  is  only 
left  long  enough  to  acquire  the  consistence  of  a  thick 
paste,  which  is  then  shaped  into  cones  or  blocks,  which 
are  allowed  to  dry  completely  in  an  airy  place.  If 
the  colours  are  to  be  sold  in  the  form  of  powder,  the 
dried  lumps  are  crushed. 

To  accelerate  drying,  the  pulp  may  be  put  through 
a  hydro-extractor,  or  dried  in  hot-air  stoves  or  rooms. 
As,  however,  this  last  method  entails  special  appliances 
and  also  expenditure,  this  acceleration  is  only  resorted 
to  when  rendered  necessary  by  special  conditions. 

The  Hydro-extractor. — When  a  substance  is  set  in 
rapid  rotation,  it  tends  to  fly  away  from  the  centre  at 
which  the  rotational  force  is  applied.  The  centrifugal 
force  thus  coming  into  action  increases  with  the  velocity 
of  rotation  and  with  the  distance  of  the  substance 
from  the  axis  of  rotation. 

The  centrifugal  hydro-extractor  consists,  therefore, 
of  a  vessel  in  rapid  rotation;  and  if  a  liquid  be  intro- 
duced into  such  vessel,  it  is  projected  with  considerable 
force  against  the  peripheral  walls.  If  the  peripheral 
surface  be  perforated,  the  liquid  portion  of  a  charge 
consisting  of  liquid  and  solid  matters  will  be  ejected 
through  the  perforations,  while  the  solid  matter  remains 
inside.  As  a  rule,  a  few  minutes'  treatment  in  a  hydro- 
extractor  is  sufficient  to  separate  the  water  from  a 
thin  pulp  so  completely  that  the  solid  residue  is  in  an 
almost  completely  dry  state.  A  hydro-extractor  which, 


though  of  an  old  pattern,  is  well  adapted  for  the  purposes 
of  the  colour-maker,  is  shown  in  Fig.  16. 

The  drum  A,  which  revolves  easily  on  a  vertical 
axis,  is  of  metal,  and  is  provided  with  a  large  number 
of  fine  perforations  on  its  peripheral  surface.  It  can 
be  rotated  at  high  speed  by  means  of  the  crank  /  and 


FIG.  1 6. 

pinions  d,  e,  or  by  the  fast-and-loose  pulley  a  b  con- 
nected with  a  source  of  power.  To  prevent  any  of 
the  charge  from  being  projected  over  the  rim  of  the 
drum,  the  upper  edge  is  turned  over  so  as  to  leave 
only  a  comparatively  small  opening  at  the  top.  The 
lower  end  of  the  drum  shaft  carries  a  strong  steel 
spindle,  which  must  be  carefully  machined  and  enable 
the  drum  to  revolve  as  easily  as  possible.  This  is 


essential,  because  even  small  machines  require  a  com- 
paratively large  amount  of  motive  power — which  is 
not  surprising  in  view  of  the  high  speed  at  which  the 
drum  has  to  revolve  in  order  to  perform  its  functions. 

The  drum  is  enclosed  in  a  casing  of  somewhat  larger 
diameter,  which  may  be  of  any  convenient  material. 
The  bottom  of  the  casing  is  preferably  tapered  slightly 
downward,  and  is  covered,  at  its  lowest  part — below 
the  bearing  of  the  drum — with  a  sieve  communicating 
with  a  pipe  through  which  the  ejected  liquid  is  drained 

When  a  liquid,  containing  solid  matter,  is  fed  into 
the  drum,  which  is  already  running  at  high  speed,  the 
liquid  is  thrown,  by  the  centrifugal  force,  against  the 
peripheral  surface  of  the  drum  and  escapes  through 
the  perforations,  leaving  the  solid  matter  behind. 
Where  large  crystals  are  in  question,  as  for  instance  in 
sugar  factories,  the  centrifugal  machine  can  be  employed 
without  any  additional  precautions,  the  liquid  being 
expelled  and  the  crystals  being  practically  dried  by 
keeping  the  machine  running  a  short  time  longer. 
In  the  case  of  the  pulp  obtained  by  levigating  colours, 
however,  this  procedure  would  result  in  failure,  because 
the  fine  solid  particles  would  be  ejected  along  with  the 
liquid  and  the  drum  would  be  left  quite  empty. 

In  this  case  it  is  therefore  necessary  to  provide  means 
for  retaining  the  solid  matter  in  the  drum,  and  allow 
only  the  water  to  escape,  with  which  object  the  drum 
is  lined  with  a  bag  of  closely  woven  fabric,  open  at 
the  top  and  fitting  snugly  against  the  inner  surface 
of  the  drum.  When  the  drum  is  first  started,  the 
ejected  liquid  is  milky,  no  fabric  being  sufficiently  close 
to  retain  all  the  extremely  fine  solid  particles  present. 
In  a  very  short  time,  however,  the  liquid  will  begin 


FIG.  17. 


to  run  away  perfectly  clear,  this  occurring  as  soon  as 
the  pores  in  the  fabric  have  become  so  far  obstructed 
by  the  projected  solids  as  to  allow  water  alone  to  pass 
through.  The  milky  water  is  then  returned  to  the 
feed  tank  and  run  slowly  into  the  machine.  The  water 
is  very  quickly  expelled,  and  the  colour  remains  in  the 
drum  as  a  stiff  paste,  of  sufficient  consistence  to  be 
moulded  into  lumps  of  any  desired  shape.  The  use 
of  the  hydro-extractor  may  be  particularly  recom- 
mended when  ample  motive  power  is  available  and 
accelerated  draining  is  desirable. 

Fig.  17  illustrates  a  modern  type  of  hydro-extractor 
with  bottom  discharge  and  suspended  drum,  the  shaft 
of  which  is  coupled  directly  to  an  electro-motor. 

Filter-presses. — Whereas  the  hydro-extractor  is 
only  used  in  particular  cases  for  the  purpose  of  the 
earth-colour  manufacturer,  the  filter-press  enjoys  more 
extensive  application.  Every  filter-press  is  composed  of 
a  number  of  closely  fitting  press  frames,  held  together  by 
the  pressure  of  a  screw.  These  frames,  when  assembled, 
form  chambers  provided  with  inlet  and  outlet  openings. 
vSuitably  shaped  and  stitched  filter -cloths  are  secured 
inside  the  chambers,  and  the  sludge  to  be  filtered  is 
run  into  the  press  from  a  high-level  tank.  The  water 
passes  through  the  filter -cloths  and  runs  off,  whilst 
the  colour  earth  gradually  fills  the  chambers.  When 
draining  is  completed,  the  press  is  taken  apart  and 
emptied.  In  this  way  the  earths  are  obtained  in  the 
form  of  more  or  less  dry  cakes,  which  are  then  put 
through  further  treatment  or  dried. 

Fig.  1 8  shows  a  Dehne  filter-press  suitable  for  the 
earth-colour  manufacturer.  Wood  internal  fittings 
are  often  used,  because  wood  does  not  affect  the  shade 
of  the  colours ;  but,  wherever  the  nature  of  the  materials 


admits,  iron  presses  are  to  be  preferred  on  account 
of  their  greater  durability  and  the  certainty  of  the 
joints  continuing  tight.  The  finer  the  grain  of  the 
levigated  colour,  the  more  difficult  the  expulsion  of 
the  water;  but  as  a  rule,  a  pressure  of  115-195  inches, 
water-gauge,  will  be  sufficient. 

If  the  sludge  be  run  into  the  press  from  a  tank  at 
sufficient  height,  two  charges  can  be  worked  in  a  day, 
but  the  cakes  will  not  be  as  firm  as  butter  of  medium 

hardness.  It  is  better  to  pump  the  charge  into  the 
press  by  means  of  a  special  diaphragm  pump.  The 
drainage  is  then  incomparably  quicker,  the  cakes  will 
be  formed  in  about  an  hour  and  will  also  be  drier.  A 
good  deal,  however,  depends,  naturally,  on  the  nature 
of  the  earth  colour. 

If  the  colour  contains  acid,  alkali  or  salts,  the  filter- 
cloths  can  be  washed  by  flushing  the  press  with  water 
under  pressure.  The  cloths  are  made  of  specially 
fine  cotton  fabric.  The  press-runnings,  which  are 
never  quite  clear,  are  collected  in  a  clarifying  tank, 


where  they  are  treated  with  lime  and  kieserite,  whereby 
gypsum  is  formed,  and  the  mass  is  put  through  a  filter- 
press,  which  retains  the  solids  and  leaves  the  effluent 

Filter-cloths  which  have  become  choked  by  use  are 
spread  on  a  table  and  scrubbed  with  water,  or  else 

FIG.  19. 

washed  in  a  special  machine  (Fig.  19),  consisting  of  a 
rotary  drum,  with  belt  drive,  the  rotation  circulating 
the  water  in  the  interior  trough  and  enabling  it  to 
extract  the  dirt  from  the  cloths.  The  flow  and  dis- 
charge of  the  water  are  controlled  by  valves,  and  the 
water  may  be  warmed  by  admitting  steam  into  the 
machine.  The  size  of  the  washer  depends  on  that 
of  the  filter-cloths. 

From  the  press,  the  cakes  of  colour  are  conveyed 


to  the  drying-plant,  usually  by  the  aid  of  automatic 

Drying  Appliances. — The  stiff  paste  or  cakes  from 
the  hydro-extractor  or  filter-press  can  be  shaped,  but 
require  to  be  dried  before  they  are  put  on  the  market. 
Drying  is  a  wearisome  operation,  the  finely  divided 
material  taking  a  very  long  time  to  dry  completely, 
even  during  the  summer  months,  whilst  in  winter  it 
is  almost  impossible  to  get  certain  colours — such  as 
ferric  oxide  colours  and  levigated  clay — quite  dry  in 
the  air,  the  inside  of  the  lumps  remaining  soft  and 
pasty  after  lying  for  months. 

The  only  way  in  which  this  troublesome  delay  in 
the  completion  of  the  operation  can  be  overcome  is 
by  artificial  dr}dng;  but  as  the  employment  of  arti- 
ficial heat  entails  expense,  it  is  necessary  to  carry  on 
the  process  with  the  smallest  possible  outlay,  in  view 
of  the  low  commercial  value  of  most  earth  colours. 

Long  experience  has  convinced  the  author  that  the 
arrangement  of  the  drying-rooms  in  many  colour  works 
is  based  on  entirely  wrong  principles,  and  that  a  great 
portion  of  the  heat  furnished  by  the  fuel  is  wasted. 
For  this  reason  the  description  of  a  properly  arranged 
drying-room  will  be  welcomed  by  a  number  of  readers. 

It  is  a  well-known  fact  that  hot  air  is  lighter  than 
cold.  Consequently,  when  a  room  is  artificially  heated, 
the  highest  temperature  will  be  found  just  under  the 
roof  or  ceiling,  and  articles  placed  in  that  part  of  a 
heated  room  will  dry  much  faster  than  those  near  the 
floor.  If  the  drying-room  is  heated  by  an  ordinary 
stove,  articles  placed  on  a  fairly  low  level  will  only 
dry  very  slowly,  because  the  hot  air  flowing  from  the 
stove  tends  to  ascend. 

In  order,  therefore,  to  utilise  the  entire  space  of  the 


drying-room,  it  is  necessary  to  place  the  heating  appar- 
atus in  such  a  position  that  the  whole  of  the  room  will 
be  warmed  as  uniformly  as  possible.  The  stove  should 
therefore  be  situated  in  a  chamber  underneath  the 
drying-room  proper. 

Because  air  that  is  already  saturated  with  moisture 
cannot  take  up  any  further  quantity,  care  must  be 
taken  to  remove  the  damp  air  continuously  from  the 
drying-room,  and  to  replace  it  by  dry  air.  This  may 
be  effected  by  suitably  designed  ventilation,  on  the 
lines  shown  in  Fig.  20,  which  represents  a  drying-room 
arranged  in  such  a  way  as  to  provide  for  all  the  above- 
mentioned  contingencies,  and  ensure  continuous  drying. 

The  heating  apparatus  is  located  in  the  cellar,  and 
consists  preferably  of  a  slow-combustion  stove  com- 
prising a  cast-iron  cylinder,  with  an  air  inlet  A  (with 
sliding  regulator  T),  for  the  air  of  combustion,  and 
a  shoot  F  at  the  top,  through  which  the  stove  is  fed 
with  fuel — preferably  coke,  on  account  of  its  great 
heating  power. 

The  stove  is  surrounded  by  an  iron  or  brick  shell  M, 
having  two  flues  R  and  Ri  leading  to  the  chambers 
I  and  II,  where  they  terminate  in  register  cowls  K, 
which  can  be  adjusted,  by  turning  the  handles  h,  so 
that  when  the  slots  o  in  K  coincide  with  corresponding 
slots  in  the  end  of  the  pipe,  the  maximum  amount  of 
hot  air  from  the  stove  is  delivered  into  the  drying- 
chambers;  and,  by  suitably  adjusting  the  cowls  and 
the  draught  through  the  fire-door  T,  it  is  possible  to 
regulate  the  temperature  of  the  chambers  to  within 
one  degree  of  the  thermometer  scale.  When  only  one 
of  the  drying-chambers  is  required  to  be  heated,  the 
register  in  the  other  is  closed,  and  the  whole  of  the  hot 
air  is  delivered  to  the  first  one.  With  this  arrangement 


none  of  the  heat  is  wasted,  and  the  contents  of  one 
chamber  can  be  dried  while  those  of  the  other  are  being 
removed  and  replaced. 

The  moisture-laden  air   from  the   drying-chambers 
can  be  led  direct  into  the  stove  chimney.     When  coke 

FIG.  20. 

is  used,  the  flue  gases  consist  almost  entirely  of  carbon 
dioxide.  If  the  vent  pipes  are  led  from  the  top  of  the 
drying-chambers  into  the  chimney,  the  hot  gases  ascend- 
ing the  latter  induce  a  strong  draught  in  the  chambers 
and  carry  off  the  moist  air  into  the  open.  These  pipes, 
also,  are  fitted  with  registers,  which,  when  suitably 


adjusted,  assist  in  the  maintenance  of  a  uniform  drying 

The  colours  to  be  dried  are  spread  on  trays  laid  on 
suitable  racks  in  the  drying-chambers ;  and,  by  carefully 
planning  out  the  available  space,  a  very  large  quantity 
of  colour  can  be  quickly  and  completely  dried  in  a 
comparatively  small  plant.  The  cost  of  the  fuel  is  so 
small  as  to  be  more  than  counterbalanced  by  the  saving 
of  time. 

The  heating  arrangements  in  drying-rooms  are 
capable  of  improvement  in  many  respects,  especially 
where  steam  is  at  disposal ;  and  in  such  cases,  it  is 
better  to  substitute  steam  heating  for  a  fire.  It  will 
then  be  necessary  to  put  in  a  good  fan,  or  other  device, 
to  ensure  the  removal  of  the  moist  air.  An  excessive 
room  temperature — above,  say,  50°  C.  (122°  F.)— 
is  not  only  superfluous,  but  in  many  cases  injurious, 
because,  apart  from  the  fact  that  some  colours  change 
in  shade  when  over -warmed,  an  unduly  high  tempera- 
ture causes  the  surface  layers  to  dry  very  quickly  and 
form  a  crust  which  prevents  the  escape  of  water  vapour 
from  the  interior  of  the  material. 

Another  form  of  drying-plant  for  earth  colours  is 
the  drying-floor,  a  large  room  with  a  rammed  concrete 
or  stone  floor,  intersected  with  brick  flues  (about  one 
foot  square),  covered  with  iron  or  concrete  slabs  and 
conveying  hot  flue  gases  from  a  furnace.  These  floors 
are  particularly  suitable  where  there  is  a  possibility 
of  utilising  an  existing  supply  of  hot  flue  gases. 

Drying-tunnels  are  specially  adapted  where  large 
amounts  of  material  have  to  be  dried.  The  tunnels 
are  built  of  brick  and  provided  with  a  rail  track  on 
which  the  trucks  carrying  a  series  of  trays  laden  with 
colour  are  run.  As  the  trucks  move  slowly  forward, 


they  are  met  by  a  current  of  hot  air  which  dries  the 
charge.  The  tunnel  is  kept  filled  with  laden  trucks, 
each  fresh  one  introduced  pushing  a  finished  one  out 
at  the  further  end. 

In  many  cases,  drying  troughs  are  also  useful.  These 
are  long,  semicircular,  jacketed  troughs  of  boiler  plate, 
hot  air  or  steam  being  passed  through  the  jacket  space. 
A  worm  conveyor  keeps  the  contents  moved  forward, 
turned  over  and  mixed  to  facilitate  drying. 

Mention  may  finally  be  made  of  vacuum  drying- 
cupboards,  which  are  heated,  air-tight  chambers,  for 
the  material,  in  which  the  air  is  partially  exhausted, 
thus  increasing  the  rate  of  evaporation  of  the  water 
and  causing  the  materials  to  dry  quickly  at  a  much 
lower  temperature  than  otherwise. 


The  distributing  and  covering  power  of  the  earth 
colours  depends — apart  from  their  special  properties— 
on  the  fineness  of  their  particles.  For  this  reason, 
all  the  means  adopted  for  the  purpose  of  pulverisation 
are  of  particular  interest.  The  most  important  crush- 
ing and  powdering  devices  have  already  been  described, 
and  may  be  referred  to,  all  that  needs  mention  in 
addition  being  the  fact  that  stone  mills  also  are  used 
for  fine  grinding. 

The  ground  products,  however,  are  not  entirely 
homogeneous,  always  containing,  in  addition  to  the 
very  finest  particles,  those  of  a  coarser  nature  which 
must  be  removed  by  sifting. 

Sifting  machines  are  essentially  sieves  through  which 
the  colour  is  passed.  The  sieves  are  made  of  wire 
gauze  or  bolting-cloth,  stretched  on  prismatic  frames 


which  are  rotated  (centrifugal  sieves),  or  superposed 
on  the  flat  and  reciprocated.  In  centrifugal  sieves, 
the  material  is  projected  against  the  sieve,  and  the 
whole  apparatus  is  in  a  state  of  vibration,  or  else  beaters 
are  provided  to  keep  the  fine  orifices  in  the  sieve  from 
choking  up. 

Nowadays    there    are    numerous    types    of    sifting 

FIG.  21. 

devices,  none  of  which,  however,  can  be  considered  as 
the  best  for  all  purposes,  since  each  type  of  earth  colour 
behaves  differently  and  requires  special  treatment. 
The  proportion  of  moisture  in  the  material,  also,  has 
an  important  influence  on  the  method  of  treatment 

A  typical  flat  sif ting-machine,  with  eccentric  jig 
motion,  is  illustrated  in  Fig.  21.  The  machine  is 
fed  through  a  hopper  provided  with  feed  rollers,  the 


rate  of  feed  being  adjustable.  The  screened  product 
is  discharged  through  a  shoot  at  one  side  of  the  machine, 
and  the  residue  at  the  opposite  side,  into  boxes,  etc., 
placed  underneath. 

For  materials  that  give  off  a  large  amount  of  dust, 
the  machine  can  be  enclosed  in  a  dust-proof  casing, 
in  which  event  the  product  and  residue  are  delivered 

FIG.  22. 

into  drawers.  The  machine  is  easily  cleaned  and  the 
sieves  quickly  changed,  and  is  well  adapted  for  dealing 
with  a  succession  of  different  materials.  The  hopper 
can  be  fitted  with  a  pair  of  adjustable  crushing 

Fig.  22  is  a  drum  sifter,  which  is  fed  by  means  of  a 
hopper  and  worm ;  and  the  drum  can  be  covered  with 
wire  or  silk  gauze.  The  sifted  product  falls  into  a 
worm  conveyor  in  the  bottom  of  the  casing  and  is 



discharged  at  the  side.  This  may  be  replaced  by  a 
series  of  mouths  for  discharging  direct  into  bags,  or  the 
machine  can  be  adapted  to  deliver  into  an  elevator, 
worm  conveyor  or  other  means  of  transport  to  a 

The  screenings  are  discharged  through  a  shoot  at 
the  back  of  the  machine,  and  can  be  handled  in  various 

FIG.  23. 

ways.     A  beater  is  provided  to  clear  the  drum  and 
increase  the  output. 

Fig.  23  illustrates  a  centrifugal  sifting-machine  for 
producing  very  fine  powder  in  large  quantities  without 
any  escape  of  dust.  It  contains  a  screening  drum, 
the  frames  of  which  are  detachable  and  facilitate  chang- 
ing the  sieves.  A  beater  revolving  inside  the  drum 
projects  the  powder  against  the  sieves,  such  portions 
as  pass  through  being  taken  up  and  discharged  by  a 
worm  conveyor;  this,  however,  can  be  replaced  by  a 
bagging  device,  etc. 



Colour  earths  are  sometimes  calcined  at  a  high  tem- 
perature in  order  to  modify  their  structure  and  shade, 
the  operation  being  accompanied,  in  some  cases,  by 
the  destruction  of  organic  admixtures  and  the  expulsion 
of  volatile  constituents. 

An  important  feature  of  calcining  is  that  it  improves 
the  covering  power  of  many  colours,  especially  heavy 
spar  and  certain  ferric  oxide  pigments.  This  alteration 
is  probably  due  to  the  heat  causing  the  finest  particles 
to  cohere,  and  also  to  the  expulsion  of  chemically-com- 
bined water,  etc. 

The  change  of  shade,  which  is  often  dependent  on 
the  degree  and  duration  of  the  heating,  is  probably 
also  connected  with  cohesion ;  but  in  many  instances 
it  is  attributable  to  chemical  modifications  produced 
by  the  treatment ;  ferric  hydroxide,  for  example,  losing 
its  water  of  hyd  ration  when  heated  and  becoming 
transformed  into  ferric  oxide. 

The  details  of  the  calcination  process  vary  with  the 
nature  of  the  material,  and  will  therefore  be  described, 
together  with  the  appliances  used,  when  we  deal  with 
the  colours  which  require  to  be  put  through  this 


It  is  very  important  that  the  maker  of  earth  colours 
should  always  be  able  to  turn  out  his  products  uniform 
in  shade,  and  since  the  raw  materials  are  liable  to  vary 
in  character,  and  the  composition  of  the  earths  from 
one  and  the  same  deposit  is  not  invariable,  the  desired 
shade  has  to  be  obtained  by  mixing.  For  this  purpose, 


standard  samples  must  be  prepared,  for  comparison 
in  matching. 

Mixing  is  a  highly  important  operation,  on  the  proper 
performance  of  which  oftentimes  depends  the  sale  of 
certain  colours  and  the  reputation  of  the  maker.  It 
may  be  effected  in  various  ways,  such  as  shovelling 
the  ingredients  together  or  by  combining  the  work 
with  grinding  in  edge-runner  mills,  ball  mills,  etc. 
Another  method  is  the  mixing  barrel  shown  in  Fig.  24, 

FIG.  24. 

a  strong  cask  mounted  on  an  axial  shaft  driven  by  a 
motor,  etc.  The  barrel  is  filled  about  two-thirds 
full  of  the  materials  to  be  mixed,  and,  after  closing 
the  feed  door,  is  slowly  rotated,  since,  if  run  at  excessive 
speed,  the  contents  are  merely  projected  against  the 
sides  of  the  barrel  by  centrifugal  force,  and  it  can  then 
be  turned  for  hours  without  result.  The  mixing  effect 
can  be  considerably  increased  by  mounting  the  barrel 
so  that  the  shaft  is  offset  from  the  longitudinal  axis 
of  the  barrel  by  an  angle  of  about  30°,  the  contents 
being  then  moved  from  side  to  side  at  each  revolution 


and  thus  more  intimately  intermixed  by  the  twofold 

In  addition  to  such  home-made  appliances,  there 
are  mixing-machines  of  the  type  illustrated  in  Fig.  25, 
the  body  of  which  is  fitted  with  a  distributing  worm 
at  the  top,  and  a  pair  of  rollers  at  the  bottom.  Below 
the  rollers,  which  are  covered  by  plates  that  can  be 

FIG.  25. 

adjusted  at  a  convenient  angle,  is  a  worm  conveyor 
delivering  into  an  elevator,  outside  the  machine  casing, 
which  connects  the  two  worms.  One  or  more  discharg- 
ing-doors,  according  to  the  size  of  the  machine,  are 
provided  under  the  worm  conveyor  at  the  end  next 
the  elevator.  The  feed  hopper  can  be  arranged  on 
the  elevator  or  on  top  of  the  machine,  according  to 
local  conditions. 

In  working  this  machine,  the  elevator  and  distributing 
worm  are  started  and  the  full  charge  is  fed  into  the 


hopper.  When  it  has  all  passed  through  the  distributor 
and  is  lodged  on  the  sloping  plates  and  bottom  rollers, 
the  latter  and  the  worm  conveyor  are  set  in  motion, 
the  material  being  then  carried  through  by  the  rotation 
of  the  rollers  and  dropping  on  to  the  conveyor,  which 
delivers  it  to  the  elevator,  to  be  returned  to  the  dis- 
tributor. In  this  way  the  charge  is  kept  in  continuous 
circulation,  and  the  finely  divided  particles  are  repeat- 
edly intermingled,  a  uniform  mixture  being  obtained. 
The  effect  is  heightened  by  the  grinding  action  of  the 
rollers  as  the  material  passes  between  them. 

The  serial  order  of  the  various  ingredients,  their 
physical  condition  (granular  or  powder),  and  their 
density,  are  all  immaterial,  the  mixing  being  effected 
so  intimately  that  when,  for  example,  a  colour  is  shaded 
with  aniline  dyes,  the  ingredients  are  so  completely 
blended  in  less  than  an  hour  that  even  the  smallest 
sample  then  taken  will  perfectly  represent  the  bulk. 

These  machines  are  made  in  various  sizes,  are  entirely 
automatic,  both  in  charging,  discharging  and  mixing, 
and  are  quite  dust-proof,  the  consumption  of  power 
being  also  small.  If  necessary,  they  can  be  combined 
with  a  crusher  or  sifter  feeding  direct  into  the  hopper. 

A  simple  means  of  ascertaining  whether  the  mixing 
is  completed,  and  one  that  can  also  be  employed  for 
judging  the  character  of  ground  materials,  consists 
in  placing  a  sample  of  the  product  on  a  sheet  of  white 
paper  and  spreading  it  out,  under  gentle  pressure, 
with  a  steel  or  horn  spatula.  No  irregularities,  streaks, 
spots  or  granules  should  then  be  discernible  either  by 
the  unaided  eye  or  under  a  magnifier. 

Improving,  which  consists  in  staining  earth  colours 
with  other  (usually  organic)  colouring  agents,  to  improve 
the  shade,  is  an  operation  which  is  generally  resorted 


to  only  in  case  of  need,  because  it  means  extra  expense, 
and  is  of  no  value  unless  light-proof  colours  are  used. 
No  permanent  effect  can  be  obtained  by  merely  mixing- 
in  coal-tar  dyes  at  random.  In  addition  to  certain 
organic  dyestuffs,  artificially  prepared  mineral  colours 
and  colour  lakes — artificial  preparations  of  an  organic 
dyestuff  with  an  inorganic  substratum — are  also  used 
for  improving. 

Another  way  of  improving  earth  colours  is  by  pre- 
cipitating certain  coal-tar  dyes  on  them,  in  presence 
of  a  fixing  agent.  Of  course  the  dyes  used  must  not 
only  be  fast  to  light,  but  also  inert  towards  the  sub- 
stratum and  to  any  other  ingredient,  such  as  lime,  that 
is  subsequently  added  to  the  earth  colours. 

The  following  dyestuffs  (Hochst)  are  suitable  for 
direct  precipitation  on  siliceous  colours  (green  earths, 
clay,  ochres,  etc.). 

Auramine,  cone.  O,  I,  II ;  new  phosphine  extra ; 
chrysoidine  A  cryst.,  B  cryst.,  C  extra;  Vesuvine 
(all  marks) ;  cachou  brown  D,  G ;  dark  brown  M,  MB ; 
safranine  G,  GS  cone.,  B  cone.;  rhodamine  O  extra, 
B,  B  extra ;  fuchsine  (all  marks) ;  fuchsine  acetate ; 
cerise  G,  R;  grenadine  O,  R,  RR;  maroon  O  extra; 
new  fuchsine  O,  P ;  methylene  violet  (all  marks) ; 
peacock  blue  P ;  Victoria  blue  B,  R ;  thionine  blue  GO ; 
methylene  blue  (all  marks) ;  malachite  green  (all 
marks) ;  brilliant  green  (all  marks) ;  coal  black  O,  I,  II. 


The  colour  pulp  can  be  made  into  tablets  by  moulding 
it  in  dry  boxes  divided  into  a  number  of  compartments. 
The  colour  shrinks  in  drying,  and  the  tablets  will  then 
easily  fall  out  of  the  moulds.  Cones  are  obtained  by 


placing  the  pulp  in  a  box,  the  bottom  of  which  is  per- 
forated with  numerous  holes  of  uniform  size,  the  box 
being  then  tapped  against  the  surface  of  a  stone  table. 
At  each  stroke,  a  certain  amount  of  colour  is  forced, 
in  the  shape  of  small  cones,  through  the  perforations, 
on  to  a  sheet  of  paper  underneath.  The  cones  are 
then  dried. 

Some  colours  are  moulded  into  blocks  by  forcing 
the  partly  dried  paste  into  suitable  moulds — preferably 
of  metal,  so  that  they  may  be  engraved  with  the  maker's 
name,  or  other  imprint — and  left  to  dry  slowly  and 
without  cracking.  The  cakes  may  be  prevented  from 
crumbling  by  incorporating  a  small  quantity  of  adhesive, 
such  as  a  weak  solution  of  dextrin,  with  the  water  in 
which  the  colour  is  suspended. 



THE  white  earth  colours  are  important  for  the 
purposes  of  the  colour-maker,  because  not  only  are 
they  used  by  themselves  as  paints,  but  also  serve  in 
the  production  of  light  shades  of  other  colours. 

The  white  colours  containing  clay  or  lime  are  the 
most  abundant  and  important  of  all,  and  will  therefore 
be  described  first.  The  lime  colours  comprise  caustic 
lime,  carbonate  of  lime  (chalk  or  powdered  limestone), 
gypsum  and  bone  ash. 


Though  this  product  is  not  used  direct  as  a  painters' 
colour,  it  is  employed  in  the  preparation  of  compounds 
that  are  so  used.  It  is  made  on  a  large  scale  for  the 
preparation  of  mortar,  and  there  is  therefore  no  need 
for  the  colour-maker  to  manufacture  it  himself,  since 
it  can  always  be  bought  from  a  lime-burner.  It  must 
be  borne  in  mind,  however,  that  lime  for  the  colour- 
maker's  purposes  must  possess  certain  properties, 
failing  which  it  is  of  no  use  to  him.  What  these 
properties  are  and  how  the  product  is  made  will  now 
be  briefly  described. 

When  carbonate  of  lime,  i.  e.  native  limestone,  is 



exposed  to  strong  heat  it  parts  with  carbon  dioxide 
and  is  transformed  into  burnt  or  caustic  lime. 

CaCO3  CaO        +        CO2 

Carbonate  Caustic  Carbon 

of  lime.  lime.  dioxide. 

The  limestone  is  burned  either  in  kilns  of  very  simple 
construction,  or  else  in  more  complicated  furnaces 
in  which  a  continuous  process  is  maintained.  The 
ordinary  limekiln,  which  can  be  found  in  many  parts 
of  the  country,  consists  merely  of  four  walls,  with  a 
door  in  the  front  one  for  the  introduction  of  the  fuel. 
Kilns  of  this  kind  are  usually  set  up  in  the  vicinity  of 
the  limestone  deposits,  and  are  abandoned  when  they 
get  worn  out. 

The  limestone  is  broken  to  lumps  of  fairly  even 
size,  about  as  large  as  a  man's  head,  and  these  are  piled 
up  in  a  domed  heap  in  the  kiln,  sufficient  space  being 
left  between  the  lumps  for  the  passage  of  the  flame. 
A  fire  is  then  lighted  under  the  pile,  pine  wood  being 
mostly  used  for  this  purpose  on  account  of  its  high 
content  of  resin,  which  gives  a  very  strong  flame.  The 
fire  is  kept  up  until  the  top  of  the  pile  has  become 
white  hot,  and  only  a  blue,  smokeless  flame  is  visible. 
The  appearance  of  this  denotes  that  the  burning  is 
ended,  the  fire  being  then  allowed  to  die  out  and  the 
lumps  left  until  cool  enough  to  be  taken  out  of  the  kiln. 

This  operation  is  performed  with  great  care,  par- 
ticular importance  being  attached  to  preserving  the 
lumps  as  intact  as  possible  and  preventing  the  formation 
of  dust,  which  is  of  little  value.  The  lime  made  in  this 
way  is  endowed  with  properties  that  render  it  valuable 
for  the  purposes  of  the  colour-manufacturer;  but,  on 
the  other  hand,  possesses  certain  disadvantages. 


Owing  to  the  use  of  wood  as  fuel,  the  caustic  lime 
obtained  in  this  way  is  usually  a  very  pure  white, 
because  the  burning  is  continued  until  the  whole  mass 
is  glowing  and  the  firewood  has  been  completely 
consumed.  If  this  is  not  the  case,  the  burnt  lime  is 
greyish  in  colour,  from  the  finely  divided  particles  of 
carbon,  which,  of  course,  spoils  the  lime  for  colour- 
making.  The  defects  existing  in  lime  burned  in  the 
above  type  of  kiln  originate  in  the  irregular  character 
of  the  product.  It  will  be  evident  that  the  limestone 
lumps  that  are  nearest  the  fire  will  be  far  more  strongly 
heated  than  those  in  the  upper  part  of  the  dome ;  and 
when  calcined  lime  is  kept  incandescent  for  a  long  time, 
it  becomes  so  compact  in  texture  that  it  quenches  with 
great  difficulty  when  brought  in  contact  with  water. 
This  condition  is  known  as  "  dead  burnt,"  and  such 
lime  is  of  little  value. 

The  lumps  at  the  top  of  the  pile  are  least  exposed 
to  the  heat,  and  very  often  still  contain  carbonate,  as 
is  shown  by  the  effervescence  produced  on  treatment 
with  an  acid.  Such  lime  is  imperfectly  burnt,  and 
the  lumps  frequently  still  exhibit  the  crystalline 
structure  of  limestone  when  broken.  They  quench 
rapidly,  but  when  mixed  with  a  little  extra  water,  the 
mass  is  no  longer  of  the  buttery  consistency  typical 
of  caustic  lime,  but  contains  gritty  portions  consisting 
of  unaltered  limestone. 

Owing  to  the  defects  of  dead  burning  on  the  one 
hand  and  insufficient  calcining  on  the  other,  colour- 
makers  now  prefer  lime  that  has  been  burned  in  con- 
tinuous kilns,  because,  when  properly  made,  such  lime 
is  very  uniform  in  character,  and  is  also  cheaper  than 
that  burned  with  such  an  expensive  fuel  as  wood.  In 
consequence  of  the  greater  capacity  of  the  continuous 


kiln,  and  the  more  uniform  character  of  the  product, 
the  old-fashioned  kilns  are  more  and  more  falling  into 

The  arrangement  of  the  continuous  kiln  is  very 
simple.  The  kiln  consists  of  a  fairly  high  shaft,  open 
at  the  top,  and  provided  at  the  bottom  with  a  small 
hole  for  the  removal  of  the  burnt  lime.  A  coal  fire  is 
lighted,  and  as  soon  as  the  kiln  is  heated  up,  alternate 
layers  of  limestone  and  sufficient  coal  for  burning  it 
are  introduced.  The  burnt  lime  sinks  to  the  bottom 
of  the  shaft  and  is  pulled  out,  with  iron  hooks,  from 
time  to  time. 

Given  the  right  proportions  of  coal  and  limestone, 
the  lime  made  in  these  kilns  is  burnt  to  just  the  right 
degree,  and  is  excellent  for  builders'  use.  In  many 
cases,  however,  it  is  less  valuable  to  the  colour-maker, 
and  in  some  quite  useless.  For  example,  when  the 
coal  is  not  completely  consumed,  carbon,  even  though 
only  a  very  small  quantity,  is  deposited  on  the  lime, 
and  the  burnt  lime,  instead  of  being  a  brilliant  white, 
as  it  should  be,  is  grey ;  and  colour  made  therefrom  is 
also  greyish  white  and  will  spoil  the  shade  of  other 
colours  with  which  it  is  mixed. 

The  chemical  composition  of  the  original  limestone 
also  has  an  influence  on  the  character  of  the  burnt 
lime.  Limestone  consisting  entirely  of  carbon  dioxide 
and  lime  is  so  rare  that  sufficient  is  never  available 
for  making  burnt  lime  on  a  large  scale.  Even  the 
purest  limestone  found  native  in  large  quantities— 
namely  marble — is  not  pure  carbonate  of  lime,  but 
contains  a  certain  proportion  of  extraneous  substances. 
At  the  same  time  it  is  too  expensive  to  use  for  technical 

The  ordinary  impurities  present  in  limestone  are 


ferrous  oxide,  ferric  oxide,  magnesia  and  organic 
matter.  The  presence  of  ferrous  oxide  can  usually  be 
detected  by  the  greenish  tinge  of  the  raw  limestone, 
and  the  reddish  cast  of  the  burnt  product.  Ferric 
oxide  is  revealed  by  its  reddish  colour,  in  both  the 
limestone  and  burnt  lime. 

Magnesia,  which  is  present,  for  example,  in  dolomitic 
limestone,  cannot  be  detected  by  the  colour,  either 
before  or  after  burning,  this  oxide  being  itself  perfectly 
white ;  but  its  presence  is  a  drawback  because  if  in 
large  quantity  it  makes  the  lime  very  difficult  to  quench, 
and  such  lime  is  never  of  a  fatty  character. 

Organic  matter  betrays  itself  by  the  colour,  the 
lime  being  dark  tinted,  varying  from  grey  to  black. 
Black  limestones  usually  contain  carbon  in  an  extremely 
fine  state  of  division,  and  are  quite  useless  to  the  colour- 
maker  owing  to  the  impossibility  of  completely  burning 
off  this  contained  carbon,  which  always  imparts  a 
greyish  tinge  to  the  burnt  lime.  The  behaviour  of 
limestones  in  this  respect  varies,  however,  considerably, 
and  can  only  be  ascertained  with  certainty  by  a  trial 
burning.  Many  that  are  rather  dark  in  colour  will, 
nevertheless,  burn  perfectly  white,  whereas  others, 
much  lighter  in  shade,  always  give  a  product  that  is 
not  quite  pure  in  tone.  This  divergent  behaviour  seems 
to  have  some  connection  with  the  chemical  composition 
of  the  organic  matter  in  question.  If  it  consists  of 
coal,  or  substances  analogous  thereto,  no  really  pure 
white  lime  can  be  obtained  from  a  light  grey  limestone, 
it  being  impossible  to  burn  off  the  finely  divided  carbon 

In  addition  to  making  a  trial  burning  with  a  fairly 
large  sample  of  material,  the  behaviour  of  a  limestone 
towards  hydrochloric  acid  will  afford  some  information 


as  to  the  nature  of  the  grey  colouring  matter.  If  the 
limestone  dissolves  completely  when  suffused  with  the 
acid,  the  indications  are  favourable  for  its  usefulness 
to  the  colour-maker.  If,  on  the  contrary,  a  black 
residue  is  left,  the  coloration  is  due  to  finely  divided 
carbon,  and  there  is  then  little  prospect  of  the  material 
furnishing  a  suitable  product.  In  any  event,  a  trial 
burning  is  the  most  reliable  guide.  In  addition  to 
carbon,  the  presence  of  any  large  proportion  of  ferric 
or  ferrous  oxide  is  objectionable,  since,  in  either  case, 
the  product  will  be  tinged  red  with  ferric  oxide,  into 
which  the  ferrous  oxide  is  transformed  at  calcination 

In  addition  to  comparing  the  colour  of  the  product 
with  a  standard  sample,  the  suitability  of  a  burnt  lime 
for  colour-making  can  be  tested  by  quenching.  If  a 
lump  about  the  size  of  the  fist  be  placed  in  a  large 
porcelain  basin  and  suffused  with  a  small  quantity  of 
water,  preferably  poured  in  a  thin  stream,  the  lime, 
if  properly  burned,  will  continue  to  absorb  the  water 
for  a  considerable  time,  like  a  sponge,  and  will  very 
soon  give  evidence  of  a  brisk  reaction  by  increasing 
in  bulk  and  generating  such  an  amount  of  heat  as  to 
cause  the  immediate  evaporation  of  a  few  drops  of 
water  allowed  to  fall  on  the  surface  of  the  mass. 
Finally,  the  entire  lump  will  crumble  down  to  a  very 
delicate,  voluminous  powder,  consisting  of  slaked  lime 
(calcium  hydroxide). 

This  chemical  reaction  is  expressed  by  the 
equation  :— 

CaO         +         H2O  Ca(OH)2 

Lime  Water         Calcium  hydroxide. 

When  the  amount  of  water  added  to  burnt  lime  is 


no  more  than  sufficient  to  effect  its  transformation  into 
hydroxide,  this  latter,  as  already  stated,  forms  a  deli- 
cate white  powder.  The  addition  of  more  water  results 
in  the  formation  of  a  homogeneous  pulp,  of  a  peculiar 
fatty  character.  Since  this  fatty  appearance  is  only 
possessed  by  pure  lime,  it  is  a  criterion  of  high  quality 
in  burnt  lime,  and  contrasts  strongly  with  that  of  the 
less  valued  poor  (or  lean)  lime. 

Calcium  hydroxide  acts  as  an  extremely  powerful 
base,  and  therefore  must  not  be  mixed  with  colours 
that  are  sensitive  to  the  action  of  strong  bases.  As  a 
matter  of  fact,  its  direct  use  in  painting  is  very  small. 
Of  course,  a  thin  milk  of  lime  is  used  for  whitewashing 
walls,  etc. ;  and  if  any  colouring  ingredients  are  added 
they  must  be  such — e.  g.  ochres — as  are  not  affected 
by  the  lime.  Nevertheless,  quick  and  slaked  lime  are 
very  important  in  colour-making,  as  forming  the 
originating  material  for  the  preparation  of  a  number 
of  colours. 

When  slaked  lime  is  mixed  with  sufficient  water  to 
form  a  stiff  pulp,  and  is  left  exposed  to  the  air  for  some 
time,  a  change  will  be  observed  to  take  place,  the  mass 
solidifying  gradually  (commencing  on  the  outside)  and 
finally  crumbling  to  a  soft  white  powder.  This  change 
is  due  to  chemical  action,  the  lime  having  a  great 
affinity  for  carbon  dioxide,  which  it  readily  takes  up 
from  the  atmosphere — a  fact  which  explains  the 
solidification  mentioned.  It  would  be  erroneous  to 
assume  that  the  lime  is  again  completely  converted 
into  calcium  carbonate  in  this  way ;  for,  though  such 
conversion  does  ultimately  take  place,  it  requires  a 
very  long  time  for  completion. 

The  resulting  compound  is,  actually,  a  double  com- 
pound of  calcium  oxide  and  carbonate.  Although  this 


compound  has  fairly  strong  basic  properties,  they  are, 
nevertheless,  far  weaker  than  those  of  caustic  lime, 
being  partly  neutralised  by  the  carbon  dioxide  absorbed. 
If  the  superficial  area  of  the  slaked  lime  be  increased 
by  spreading  it  out  thinly,  so  as  to  offer  greater 
opportunity  for  the  action  of  carbon  dioxide,  the 
formation  of  the  double  compound  in  question  will  be 
greatly  accelerated. 

This  double  compound  is  prepared  artificially  in 
special  works,  and  the  resulting  colours  are  put  on  the 
market  under  various  names.  They,  too,  must  not  be 
mixed  with  colours  that  are  sensitive  to  alkali,  and  on 
this  account  they  cannot  be  used  in  fine  paints.  If 
applied  as  a  white  priming  to  the  walls  of  rooms,  care 
must  be  taken  to  cover  the  coating  with  some  substance 
that  will  protect  the  topping  colour  from  the  action  of 
the  lime.  For  this  purpose,  painters  use  a  wash  of 
milk,  soap  and  water,  etc. 

An  important  property  of  lime  is  its  behaviour 
towards  casein,  the  substance  forming  the  curd  of 
milk.  With  this  body  it  combines  to  form  a  mass 
which  sets  hard -and  is  highly  resistant,  viz.  calcium 
caseate,  and  is  formed  when  limewash  is  stirred  up 
with  milk  or  freshly  precipitated  casein.  Weatherproof 
distempers  for  outside  use  are  prepared  in  this  manner. 


The  preparation  frequently  met  with  in  commerce 
under  this  name  is  nothing  more  than  a  burnt  lime  of 
great  purity.  It  is  prepared  in  the  coastal  districts 
by  burning  oyster  shells,  the  resulting  burnt  lime  being 
easily  transformed  into  a  fine  powder,  the  pure  white 
colour  of  which  is  due  to  the  absence  of  iron.  It  is 
used  in  the  same  way  as  pure  burnt  lime,  and  is  mainly 


of  interest  in  seaside  towns  where  oyster  shells  are  often 
accumulated.  It  may  be  pointed  out  that  the  name 
pearl  white  is  often  applied  also  to  pure  white  grades  of 
white  lead. 


This  colour  is  prepared  from  any  kind  of  burnt  lime 
that  is  sufficiently  pure  ;  that  is,  free  from  ferric  oxide. 
The  method  of  preparation  is  simple,  requiring  no 
special  apparatus,  and  can  therefore  be  carried  out 
wherever  suitable  lime  is  available. 

Operations  are  commenced  by  carefully  slaking 
well-burnt  lime  with  water,  a  sufficient  excess  of  which 
is  added  to  produce  a  fairly  thick  pulp.  To  accelerate 
the  absorption  of  carbon  dioxide,  the  mass  is  exposed 
to  the  air  in  thin  layers,  by  spreading  it  out  on  boards, 
so  as  to  present  a  large  surface  to  the  air.  As  soon  as 
the  pulpy  character  has  disappeared,  the  mass  is 
detached  from  the  boards,  and  is  pressed  and  kneaded, 
with  wooden  paddles,  into  prismatic  cakes  which  are 
left  exposed  to  the  air — being,  of  course,  protected 
from  the  wet — until  the  absorption  of  carbon  dioxide 
is  complete — a  condition  that  can  be  recognised  by 
the  earthy  character  of  the  product.  The  cakes  are 
then  dried,  an  operation  entailing  great  care,  since 
lightness  is  a  sign  of  good  quality,  whereas  a  damp 
product  is  very  heavy. 

In  forming  the  cakes  they  must  not  be  touched  by 
the  bare  hands,  because  the  lime  is  so  caustic  that  it 
would  soon  destroy  the  skin.  The  foregoing  method  of 
manufacture  is  capable  of  many  improvements,  which 
can  be  introduced  without  adding  much  to  the  cost  of 
If  the  lime  is  formed  into  large  blocks,  it  will  evidently 


take  a  long  time  for  the  mass  to  acquire,  all  through, 
the  earthy  character  indicating  combination  with 
carbon  dioxide.  This  drawback  can  be  easily  remedied 
by  forming  the  mass  into  small  cakes,  which  will 
become  ripe,  owing  to  their  larger  surface,  much  sooner 
than  the  bigger  blocks. 

A  very  good  plan  to  adopt  in  moulding  is  to  form  the 
burnt  lime  into  a  stiff  paste  with  water,  preferably  by 
adding  enough  water  to  make  a  viscous  mass,  and 
leaving  this  in  a  lime-pit  for  several  weeks,  the  prolonged 
storage  enabling  the  lime  to  acquire  the  already 
mentioned  fatty  character,  and  at  the  same  time  to 
become  highly  plastic.  Lime  treated  in  this  way  can 
be  forced  through  a  nozzle,  forming  a  cylindrical  rope, 
which  can  be  cut  by  a  knife  into  convenient  lengths 
and  left  on  boards  for  a  few  days  until  they  have 
become  firm  enough  to  stand  up  without  breaking. 
Cylinders  made  in  this  manner,  with  a  length  of  about 
four  inches  and  a  diameter  of  two  inches,  will  absorb 
carbon  dioxide  very  quickly. 

The  absorption  can  be  still  further  accelerated  by 
setting  up  the  cylinders  in  an  atmosphere  highly 
charged  with  the  gas,  for  instance  in  the  vicinity  of  a 
manure  pit,  as  they  will  then  avidly  take  up  the  carbon 
dioxide  abundantly  liberated  from  the  rotting  manure. 
Similar  acceleration  will  take  place  if  the  boards 
carrying  the  cylinders  are  placed  in  a  stable,  or  in  a 
room  where  wash  for  making  spirits  is  fermenting, 
because  large  quantities  of  carbon  dioxide  are  liberated 
in  both  places. 

Working  the  caustic  mass  by  hand  is  accompanied 
by  so  many  inconveniences  that  it  seems  highly 
desirable  to  employ  some  mechanical  moulding  device 
which  will  render  contact  with  the  wet  lime  entirely 



superfluous.  It  may  be  pointed  out  that  such  a  device 
can  also  be  advantageously  used  for  moulding  all  earth 
colours  in  paste  or  pulp  form,  and  in  particular  for 
shaping  ferric  oxide  colours  into  rods  or  small  cylinders. 
Such  a  machine  (Fig.  26)  is  composed  of  a  rectangular 
box  with  semi-cylindrical  bottom,  a  detachable  shaft 
carrying  a  sheet-metal  worm  being  arranged  in  the  box 
so  that  the  worm  is  in  contact  with  the  rounded  bottom 
and  is  continued  into  the  cylindrical  extension  of  the 

FIG.  26. 

box.  This  extension  terminates  in  a  hollow  cone,  to 
the  mouth  of  which  nozzles  of  varying  aperture  (square, 
rectangular  or  round)  can  be  attached.  A  knife, 
operated  by  hand  or  mechanical  means,  enables  the 
extruded  soft  mass  to  be  cut  into  convenient  lengths, 
which  drop  on  to  a  series  of  easy  running  rollers  in 
front  of  the  nozzle,  and  are  thereby  delivered  to  an 
endless-belt  conveyor  from  which  they  can  be  trans- 
ferred to  the  drying-boards. 

When  the  box  has  been  charged  with  the  lime  pulp 
and  the  worm  is  rotated,  the  latter  forces  the  soft  mass 
into  the  cone  and  extrudes  it  through  the  nozzle,  so 


that,  as  long  as  there  is  any  material  in  the  box,  it  is 
discharged  as  a  continuous  rope,  of  square,  rectangular 
or  cylindrical  section,  on  to  the  guide -rollers,  where  it 
can  be  cut  off  into  lengths  by  the  knife. 

A  fundamental  condition  for  the  preparation  of  a 
good  Vienna  white  is  the  employment  of  pure  raw 
material,  which  must  be  free  from  ferric  oxide  or 
earthy  impurities,  and  fully  burned.  An  excellent 
material  for  this  purpose  is  calcined  mussel  shells, 
which  furnish  a  loose,  and  at  the  same  time  very  pure, 
lime,  and  are  very  largely  used  for  lime -burning  in 
places  such  as  Holland,  where  they  are  available  in 
large  quantities. 

Vienna  white  is  not  much  used  as  a  paint  colour, 
owing  to  its  powerful  alkaline  properties  which  have 
a  destructive  effect  on  many  colours.  It  is,  however, 
largely  employed  as  a  polishing  agent,  for  which  purpose 
it  is  powdered  and  is  put  on  the  market — mostly  in 
bottles — as  Vienna  lime.  Its  very  handsome  white 
colour  and  low  price  render  it  particularly  suitable  for 
coarse  painting,  for  example  as  a  prime  coating  for 
painted  interior  walls.  To  guard  against  the  danger 
of  the  painted  decoration  being  destroyed  by  the 
alkaline  nature  of  the  white,  it  is  advisable  to  coat  the 
dried  ground  with  alum  solution,  the  alumina  of  which 
combines  with  the  lime  to  form  an  insoluble  compound 
to  which  organic  colours  adhere  well.  The  sulphuric 
acid  also  enters  into  combination  with  the  lime,  the 
resulting  gypsum  having  no  effect  on  the  paints 
subsequently  applied. 


The  name  chalk  is  used  for  a  number  of  commercial 
substances  which  differ  considerably  in  both  the 


mineralogical  and  chemical  sense.  French  chalk,  for 
instance,  is  a  mineral  belonging  to  the  steatite  group 
and,  apart  from  its  name,  has  nothing  in  common  with 
true  chalk,  except  the  white  colour,  and  even  this 
differs  altogether  from  that  of  chalk  properly  so  called. 
It  is  therefore  necessary,  in  the  interests  of  proper 
nomenclature,  to  differentiate  the  various  kinds  of 
chalk,  commencing  with  the  mineral  known  by  that 
name  to  the  chemist  and  mineralogist. 

In  chemical  composition,  true  chalk  is  calcium 
carbonate,  but  of  a  fossil  character,  for  if  chalk  dust 
be  examined  under  a  high-power  microscope,  it  will  be 
seen  to  consist  of  the  shells  of  minute  animals,  and  is 
therefore  to  be  regarded  as  fossil.  The  organic  matter 
of  the  animals  has  long  disappeared,  leaving  the 
inorganic  material,  a  very  pure  calcium  carbonate, 

Such  progress  has  been  made  that  the  zoological 
status  of  the  animals  which  inhabited  the  shells — many 
thousands  of  which  are  present  in  a  lump  of  chalk- 
has  been  identified ;  and  it  is  known  that  these  animals 
were  of  marine  type.  Fig.  27  shows  the  appearance 
of  the  animal  remains  in  Meudon  chalk  when  highly 
magnified,  the  upper  half  being  viewed  by  transmitted 
light  and  the  lower  by  reflected  light. 

Notwithstanding  the  extremely  minute  dimensions 
of  the  chalk  animalculae,  their  remains  form  rocks 
of  great  thickness  in  all  parts  of  the  world.  In  Europe 
we  find,  for  example,  extensive  chalk  formations  in 
England,  whose  Latin  name  Albion  was  bestowed  on 
account  of  the  white  chalk  cliffs  occupying  long 
stretches  of  the  coast.  The  hills  of  Champagne  consist 
almost  entirely  of  chalk;  and  Riigen,  together  with 
many  other  islands,  is  nearly  all  chalk  cliffs. 



It  is  only  in  very  rare  cases,  however,  that  chalk 
occurs  in  sufficient  purity  to  be  immediately  suitable 
for  use  as  a  pigment  or  writing-material.  For  the  most 
part  it  contains  other  minerals,  or  large  fossils,  from 
which  it  has  to  be  separated  by  mechanical  treatment. 
Nodular  flints  are  often  met  with  in  chalk,  and  many 
deposits  contain  such  large  numbers  of  the  petrified 
shells  of  the  sea  urchin  that  the  chalk  really  cannot 
be  used  as  a  pigment  at  all,  by  reason  of  the  high  cost 

of  purification.  The  only  places  where  chalk  can  be 
advantageously  worked  for  the  preparation  of  pigment 
is  where  the  mineral  is  in  a  high  state  of  purity,  and 
also  contains  only  very  few  sandy  particles.  Such 
chalk  deposits  are  worked  on  a  mining  scale,  and,  as 
a  rule,  in  the  state  in  which  the  chalk  comes  from  the 
quarry ;  it  is  in  the  form  of  a  soft  mass,  easily  scratched 
with  the  finger-nail  and  of  fairly  high  density,  owing 
to  the  considerable  quantity  of  water  with  which  it  is 
ordinarily  impregnated. 

In  order  to  convert  this  crude  chalk  into  a  product 
that  can  be  used  as  a  pigmc  nt,  it  is  first  left  to  dry 


until  the  lumps  can  be  easily  broken,  and  then  crushed 
into  small  pieces,  from  which  all  the  extraneous 
minerals,  which  occur  as  large  lumps,  are  sorted  out 
and  removed.  This  picking  process  is  important, 
especially  when  the  chalk  contains  flints,  because  these 
latter  are  very  hard  and  would  injure  the  millstones 
in  the  subsequent  grinding. 

The  lumps  of  chalk  are  reduced  by  mechanical  means, 
such  as  a  stamp-mill,  or,  more  frequently,  in  a  mill  of 
the  same  type  as  for  grinding  flour,  since  it  is  impossible 
to  get  the  lumps  so  dry  as  to  produce  the  degree  of 
brittleness  necessary  for  a  thorough  reduction  in  a 
stamp-mill.  The  millstones  are  enclosed  in  a  wooden 
casing,  and  the  chalk  is  ground  in  admixture  with 
water,  the  ground  mass  escaping,  through  an  opening 
in  the  casing,  as  a  thick  pulp  which  is  stored  for  a 
considerable  time  in  large  tanks. 

Experience  has  shown  that  this  method  of  prolonged 
storage  in  contact  with  water  greatly  improves  the 
colour.  The  only  explanation  of  this  fact  is  that  the 
chalk  still  contains  a  very  small  amount  of  organic 
matter,  which  gradually  decomposes  in  presence  of 
water.  The  evidence  in  favour  of  this  is  the  peculiar 
smell  given  off  during  storage. 

Even  with  the  most  careful  grinding,  chalk  cannot 
be  transformed  into  such  a  fine  powder  that  is  directly 
fit  for  all  purposes;  and  the  only  way  to  obtain  the 
requisite  fineness  is  by  levigation.  Owing  to  the  large 
quantities  that  are  usually  handled  in  this  process,  the 
milky  liquid  coming  from  the  mill  is  mostly  run  into 
large  brick  tanks,  where  it  is  left  to  settle  until  all  the 
chalk  has  deposited  and  the  supernatant  water  is 
perfectly  clear.  Tapping-off  being  usually  imprac- 
ticable, the  water  is  generally  drawn  off  by  careful 


syphoning,  so  as  not  to  disturb  the  fine  sludge  at  the 
bottom  of  the  tank. 

The  deposit  in  the  settling-tanks  is  shovelled  into 
wooden  boxes,  perforated  at  the  sides  to  enable  the 
water  to  drain  away,  the  chalk  being  prevented  from 
escaping  by  lining  the  boxes  with  linen  cloths.  The 
pulp  soon  loses  its  liquid  character  and  shrinks  con- 
siderably, the  boxes  being  then  filled  up  with  more 
sludge,  and  so  on  until  the  contents  have  ceased  to 
shrink.  When  the  mass  is  so  far  dry  that  it  will  no 
longer  run  when  lifted,  the  boxes  are  covered  with 
boards  and  inverted,  discharging  the  contents  on  to 
the  boards,  on  which  the  mass  is  left  to  become  quite 
dry.  Filter-presses  are  also  used. 

Large  prismatic  masses  of  chalk  never  dry  so 
uniformly  as  to  prevent  the  formation  of  cracks,  and 
if  the  chalk  is  to  be  sold  in  this  form  the  cracks  are 
plastered  up  with  thick  pulp ;  this  operation,  however, 
being  superfluous  when  the  chalk  is  to  be  sold  as 

In  order  to  obtain  a  more  compact  product  and 
accelerate  the  drying  of  the  moulded  lumps,  some 
makers  use  presses,  in  which  the  fairly  dry  chalk  is 
subjected  to  progressive  heavy  pressure. 

Owing  to  the  fineness  of  the  component  particles  of 
chalk,  they  adhere  so  firmly  together,  without  any 
bind,  that  a  fair  amount  of  force  is  necessary  to  break 
down  a  piece  of  perfectly  dry  levigated  chalk.  Some- 
times, however,  chalk  exhibits  the  unpleasant  property 
of  losing  its  cohesion  almost  completely  when  dry,  and 
in  such  cases  it  can  only  be  shaped  into  prisms  with 
great  trouble.  This  peculiarity  is  specially  accentuated 
when  the  chalk  contains  magnesia;  and  in  order  to 
mould  chalk  of  this  kind  into  blocks,  a  binding  agent, 


such  as  ordinary  glue,  must  be  added  to  the  water  used 
in  grinding,  care  being  taken  not  to  use  too  much,  or 
the  chalk  will  become  too  hard,  when  dry,  for  certain 
purposes,  e.  g.  as  drawing  or  writing  chalk. 

For  some  purposes,  chalk  is  sold  in  powder  form, 
and  very  high  purity  is  not  then  essential,  an  admixture 
of  magnesia  or  clay  being  harmless.  Gilders,  for 
instance,  use  large  quantities  of  chalk  for  priming 
picture  frames,  and  stir  the  chalk  up  with  a  certain 
amount  of  bind  (mostly  size),  to  give  the  particles  the 
desired  cohesion. 

The  chief  requirement  exacted  of  a  good  quality 
chalk  is  a  handsome  white  colour;  and  this  depends 
entirely  on  the  quality  of  the  raw  material,  not  on  the 
method  of  preparation.  It  is  known  that  a  substance 
quite  devoid  of  colour  will  furnish  a  perfectly  white 
powder,  because  the  colourless  particles  reflect  the 
light  in  all  directions  without  breaking  it  up  into  its 
constituent  yellow,  red  and  blue  rays.  Chalk,  too,  is 
in  reality  a  colourless  substance,  and  reflects  light  with 
greater  uniformity  in  proportion  as  the  fineness  of  the 
particles  increases.  Consequently,  when  one  has  a 
chalk  that  is  not  perfectly  white,  it  can,  nevertheless, 
be  made  to  furnish  a  very  handsome  product  by 
bestowing  great  care  on  grinding  and  levigation. 
Properly  prepared  chalk  should  be  as  fine  as  the  finest 

When  the  colour  of  the  best  grades  of  chalk  are 
compared  with  what  may  be  termed  pure  white — such 
as  that  of  white  lead,  zinc  white,  permanent  white— 
a  skilled  eye  will  always  detect  a  greyish  or  yellowish 
tinge  in  the  former,  even  if  obtained  from  the  whitest 
Carrara  marble. 

The  grey  tinge  is  due  to  the  presence  of  organic 


matter,  which  cannot  be  eliminated  by  any  known 
means,  but  which  can  be  shown  to  exist  by  the  fact 
that  when  such  chalk  is  heated  to  incandescence  in  the 
air  for  a  short  time,  the  resulting  burnt  lime  is  pure 
white,  the  organic  matter  having  been  burned  off. 
A  yellow  tinge  is  caused  by  minute  traces  of  ferric 
oxide,  which — as  also  ferrous  oxide — almost  invariably 
accompanies  calcium  carbonate ;  and  limestone  free 
from  determinable  quantities  of  these  oxides  is  of  rare 
occurrence.  Ferrous  oxide  does  not  reveal  its  presence 
in  limestone  unless  in  large  proportion,  its  pale  green 
colour  being  of  low  tinctorial  power,  whereas  ferric 
oxide,  which  is  a  very  strong  colouring  agent,  can  be 
more  readily  detected. 

To  those  who  are  engaged  in  the  manufacture  of 
white  earth  colours,  however,  it  is  quite  immaterial 
whether  a  limestone  or  chalk  contains  ferrous  oxide, 
because  that  oxide  quickly  changes  into  ferric  oxide 
in  the  finely  divided  product,  and  a  chalk  which  was 
originally  pure  white  will  become  decidedly  yellow  in 
a  short  time. 

Fortunately,  such  a  yellow-tinged  product  can  be 
rendered  perfectly  white  by  simple  means  and  at  small 
cost,  all  that  is  necessary  being  to  add  a  suitable 
quantity  of  a  blue  colouring  matter.  When  this  has 
been  done,  the  chalk  will  seem  pure  white  to  even  the 
most  skilled  eye. 

This  result  of  adding  a  blue  pigment  is  based  on  the 
well-known  physical  fact  that  certain  kinds  of  coloured 
light  produce  white  light  when  combined,  the  colours 
that  give  this  effect  being  termed  "  complementary." 
A  pure  blue  is  complementary  to  a  yellow  with  a  reddish 
cast- — e.  g.  ferric  oxide — and  therefore  a  chalk  that  is 
tinged  yellow  by  a  small  quantity  of  ferric  oxide  can 


be  changed  into  a  seemingly  pure  white  substance  by 
the  addition  of  a  blue  pigment. 

The  only  pigments  of  use  in  this  connection  to  the 
colour-maker  are  such  as  have  very  intensive  colouring 
power  and  at  the  same  time  are  low  enough  in  price. 
Such  substances  are  ultramarine,  smalt  and  coal-tar 
dyes.  Smalt  is  the  best  because  its  colour  is  unalter- 
able. In  point  of  chemical  composition,  this  substance 
is  a  very  hard  glass  coloured  blue  by  cobalt ous  oxide. 
For  improving  the  colour  of  chalk  or  any  other  white, 
the  smalt  must  be  in  an  extreme  state  of  fine  division, 
and  levigated  to  an  impalpable  powder.  Ultramarine 
can  be  used  for  the  same  purpose,  but  is  not  so 

To  ascertain  the  correct  proportion  of  blue  pigment, 
it  is  advisable  to  make  a  systematic  experiment,  which 
is  easily  performed.  Exactly  90  parts  of  the  chalk 
in  question  are  triturated  with  10  parts  of  blue  pigment 
in  a  mortar  until  the  entire  mass  has  become  a  perfectly 
uniform  pale  blue  powder,  which  contains  10%  of  the 
blue  ingredient. 

Several  samples,  each  representing  one  hundred 
parts  of  the  white  pigment  to  be  corrected  are  carefully 
weighed  out,  I  part  of  the  blue  powder  being  added 
to  the  first  sample,  2  parts  to  the  second,  3  to  the  third, 
and  so  on,  and  the  mixtures  are  compared  with  a 
standard  white  substance,  such  as  best  white  lead  or 
zinc  white,  to  see  which  most  nearly  approaches  the 
standar4  colour.  It  is  then  easy  to  calculate  how 
much  of  the  blue  requires  to  be  added  to  100  or  1000  Ib. 
of  the  material  to  be  corrected. 

The  correction  can  be  effected  in  several  ways ;  for 
instance,  by  grinding  the  blue  pigment  directly  with 
the  bulk,  by  adding  it  at  the  levigation  stage,  or  mixing 


it  with  the  dry,  finished  product.  The  first  two  methods 
are  attended  with  certain  drawbacks  which  render  it 
difficult  to  obtain  a  perfectly  uniform  product,  owing 
to  the  specific  gravity  of  the  blue  pigments  being  higher 
than  that  of  the  whites.  Consequently,  when  the  two 
are  mixed  in  presence  of  water — as  is  always  the  case 
in  grinding  and  levigation — the  heavier  blue  pigment 
settles  down  more  quickly,  and  several  strata  can  be 
clearly  distinguished  in  the  sediment.  The  upper 
layers  will  still  have  a  decided  yellow  tinge — the 
proportion  of  blue  being  too  small  for  proper  correc- 
tion-— whilst  the  next  in  order  will  be  pure  white— 
accurately  corrected — and  those  at  the  very  bottom 
will  be  decidedly  blue,  because  they  contain  the  largest 
proportion  of  the  blue  substance. 

The  most  satisfactory  results  are  obtained  by  dry 
mixing ;  and  this  can  be  successfully  practised  when  the 
colour-maker  has  a  cheap  source  of  power  (such  as 
water  power)  available.  Where/however,  costly  power 
plant  has  to  be  provided,  only  the  finest  grades  of 
white  pigments  can  be  improved  in  this  way,  the 
expense  of  labour  being  too  high  for  cheap  materials. 

As  a  pigment,  chalk  possesses  many  valuable 
properties.  The  organic  structure  of  chalk  gives  it 
high  covering  power  as  a  wash,  a  thin  layer  applied 
to  a  surface  sufficing  to  mask  the  colour  of  the  under- 
lying ground  completely.  The  lime  in  chalk  being 
combined  with  carbonic  acid,  its  basic  properties  are 
so  extensively  weakened  that  chalk  can  be  mixed  with 
even  the  most  delicate  colours  without  fear  of  their 
shade  being  affected.  A  coating  of  pure  chalk  paint 
on  any  surface  will  never  change  colour  in  the  air; 
and  on  this  account,  chalk  is  extensively  used  both  as 
an  indoor  wash  and  by  wall-paper  manufacturers. 



Many  chemical  processes  furnish  soluble  salts  of 
lime  that  constitute  a  by-product  of  little  value.  These 
salts,  however,  can  be  advantageously  utilised  for  the 
preparation  of  an  artificial  chalk  which  is  preferable 
to  the  native  article  in  many  respects.  For  instance, 
where  large  quantities  of  calcium  chloride  solution 
are  available,  and  soda  can  be  purchased  at  a  sufficiently 
cheap  rate,  they  can  be  converted  into  artificial  chalk, 
because  these  two  substances  react  on  each  other, 
forming,  on  the  one  hand,  calcium  carbonate,  which  is 
precipitated  as  a  very  delicate,  insoluble  powder,  and 
on  the  other,  sodium  chloride,  or  common  salt,  which 
remains  in  solution,  according  to  the  equation  :— 

CaCl2  +  Na2CO3  =  CaCO3  +  NaCl. 

If,  however,  these  solutions  were  mixed  together  in 
a  crude  state,  the  resulting  product  would  be  of  only 
low  value  as  a  pigment,  being  of  a  yellow  tinge  and 
never  pure  white.  This  is  due  to  the  fact  that  the 
impure  lime  salts,  being  waste  products  from  chemical 
works,  frequently  contain  fairly  large  amounts  of 
ferric  oxide,  and  the  soda  also  is  often  so  high  in  that 
impurity  that  the  colour  of  the  precipitated  chalk  is 
considerably  impaired. 

Fortunately,,  there  is  no  difficulty  in  eliminating  this 
ferric  oxide  by  chemical  means,  and  obtaining  a  product 
of  superior  colour  to  the  best  native  chalk.  This  is 
effected  by  treating  the  perfectly  neutral  lime-salt 
solution  with  calcium  carbonate,  which  causes  the 
precipitation  of  the  iron,  a  corresponding  amount  of 
lime  passing  into  solution. 

In  order  to  eliminate  the  ferric  oxide  from  the  lime- 


salt  solution  so  completely  that  not  even  the  most 
delicate  chemical  test  known  will  be  able  to  reveal 
any  trace  remaining,  the  solution  is  placed  in  a  vat 
and  stirred  up  with  finely  powdered  chalk.  If  the 
solution  contains  any  free  acid,  effervescence,  due  to 
the  liberation  of  carbon  dioxide,  will  take  place ;  and 
in  such  event  the  addition  of  chalk  is  continued  until 
the  free  acid  is  all  neutralised,  and  the  added  chalk 
sinks  to  the  bottom  undissolved.  The  chalk  should 
be  in  slight  excess,  so  that  a  decided  sediment  is  visible 
at  the  bottom  of  the  liquid  when  at  rest. 

This  deposit  is  stirred  up  again  at  intervals  with  the 
liquid  for  several  days.  When  ferric  oxide  is  present, 
the  colour  of  the  deposit  will  gradually  change  to  a 
yellowish  brown,  through  the  precipitation  of  ferric 
hydroxide  by  the  chalk;  and  in  this  way  the  final 
traces  of  iron  can  be  removed. 

The  liquid  is  then  carefully  drawn  off,  without 
disturbing  the  sediment,  and  the  soda  solution  is  run 
in  so  long  as  a  precipitate  of  calcium  carbonate  con- 
tinues to  form.  The  completion  of  the  reaction  can  be 
ascertained  by  pouring  a  small  quantity  of  the  liquid 
into  a  tall,  narrow  glass,  leaving  it  to  clarify,  adding  a 
little  more  soda  solution  and  observing  whether  any 
further  precipitate  is  produced.  On  the  other  hand, 
it  may  be  that  an  excess  of  soda  has  already  been  added 
in  the  precipitating  tank;  and  this  can  be  determined 
by  testing  a  sample  with  turmeric  paper — blotting- 
paper  soaked  in  a  solution  of  the  colouring-matter  of 
turmeric  root — which  is  turned  brown  by  alkaline 
reagents.  Even  in  very  dilute  solution,  soda  will  give 
this  colour  change,  and  the  test  is  therefore  very 
accurate.  The  complete  precipitation  of  the  lime  in 
the  solution  can  be  ascertained  by  passing  a  small 


quantity  through  blotting-paper  and  treating  it  with 
a  little  acid  potassium  oxalate  solution,  which,  if  lime 
be  present,  will  at  once  produce  a  strong  crystalline 
precipitate  of  calcium  oxalate,  which  is  only  very 
sparingly  soluble  in  water.  If  the  oxalate  gives 
merely  a  slight  turbidity,  the  residual  amount  of  lime 
is  so  small  that  the  process  may  be  regarded  as  complete. 

Since  carbonate  of  soda  is  usually  much  dearer 
than  the  lime-salt  liquor,  it  is  preferable  to  leave  a 
small  quantity  of  the  lime  unprecipitated.  Given 
sufficient  care  in  effecting  the  precipitation,  and 
especially  when  fairly  strong  solutions  are  used,  a 
brilliant  white  precipitate  of  calcium  carbonate  is 
obtained,  which  is  in  such  a  finely  divided  state  that 
the  minute  constituent  crystals  can  only  be  detected 
under  a  high  magnifying  power. 

This  precipitated  chalk  being  already  in  an  extremely 
fine  condition  needs  no  further  preparation,  and,  when 
washed,  is  ready  for  immediate  use,  forming  a  handsome 
pigment  with  excellent  covering  power. 

When  precipitation  is  ended,  the  deposit  is  allowed 
to  settle  down,  and  the  clear  supernatant  liquid  is 
carefully  drawn  off  so  as  not  to  disturb  the  delicate 
sediment,  which  is  then  stirred  up  thoroughly  with 
clean  water,  left  to  subside,  washed  again,  and  then 
spread  out  to  dry  on  cloths  which  are  suspended  by 
the  four  sides.  The  surplus  water  drains  away  and 
the  residue  gradually  assumes  the  consistency  of  paste, 
in  which  condition  it  can  easily  be  moulded  to  any 
desired  shape.  If  left  long  enough  to  dry  completely, 
it  forms  a  very  delicate  powder,  furnishing  a  pigment 
of  excellent  quality. 

If  this  precipitated  chalk  be  moulded  into  prisms 
for  sale,  the  blocks  are  laid  on  one  of  their  broad  sides 


until  firm  enough  to  turn  over  on  to  one  of  the  narrow 
faces,  slabs  of  gypsum  being  used  as  the  supporting 
material,  in  order  to  ensure  uniform  drying.  The 
gypsum  absorbs  water  with  avidity  and  thus  dries  the 
prisms  evenly. 

A  defect  of  these  prisms  is  their  great  fragility ;  but 
their  strength  may  be  improved  by  mixing  a  little 
very  weak  solution  of  dextrin  to  the  mass  after  the 
last  washing-water  has  been  completely  removed.  In 
drying,  the  dextrin  binds  the  material  of  the  prisms 
sufficiently  to  keep  them  from  breaking  except  under 
the  influence  of  a  fair  degree  of  force. 


As  already  mentioned,  calcium  carbonate  rarely  occurs 
in  a  perfectly  pure  condition  in  Nature;  and  chalk, 
also,  is  frequently  contaminated  by  other  minerals. 
A  variety  of  limestone  occurring  as  extensive  deposits 
in  many  places  is  that  in  which  calcium  carbonate 
is  associated  with  clay.  Sometimes  the  clay  pre- 
dominates, and  the  mineral  is  then  known  as  marl, 
being  really  a  clay  contaminated  with  chalk.  If, 
on  the  other  hand,  the  chalk  forms  the  chief  constituent, 
the  mineral  is  termed  calcareous  marl. 

Calcareous  marls  are  used  in  much  the  same  way  as 
limestone,  some  modification,  however,  being  necessi- 
tated by  the  presence  of  the  clay.  Although  limestone 
containing  a  certain  amount  of  clay  can  be  burned  in 
the  kiln,  it  yields  an  inferior  lime  that  is  of  little  use  to 
the  builder  owing  to  its  low  binding  power.  Marl 
of  a  certain  composition  finds  an  important  application 
in  the  manufacture  of  hydraulic  lime  or  cement. 

The  only  kind  of  marl  suitable  for  pigment  is  that 


containing  clay  with  very  little  colour;  and  this  is  of 
somewhat  rare  occurrence,  because  most  marls  contain 
sufficient  ferric  oxide  to  give  them  a  yellow  shade. 
Marl  that  is  fairly  free  from  ferric  oxide,  however,  can 
very  well  be  used  as  pigment ;  and  many  white  pigments 
sold  as  "  chalk  "  are  really  finely  ground  marl. 

In  accordance  with  the  general  practice,  in  the  colour 
industry,  of  giving  colours  a  great  variety  of  names, 
and  suppressing  the  real  names,  which,  so  far  as  the 
artificially  prepared  colours  are  concerned,  should 
bear  some  reference  to  their  chemical  composition, 
numerous  white  earth  colours  bear  fancy  names,  though 
really  consisting  of  chalk,  lime  (generally  marl),  or 
white  clay. 

In  France,  where  both  chalk  and  clay  are  of  frequent 
occurrence — the  soil  of  Champagne,  for  instance, 
being  all  chalky — the  manufacture  of  the  white  earth 
colours  is  extensively  practised,  and  a  large  number  are 
put  on  the  market,  usually  named  after  the  place  of 
origin,  and  consisting  of  either  calcium  carbonate  or 

The  trade  names  of  the  white  earth  colours  include 
Cologne  chalk,  Bologna  chalk,  Briancon  chalk, 
Champagne  chalk,  Blanc  de  Bougival,  Blanc  de  Meudon, 
Spanish  white,  Blanc  d' Orleans,  Blanc  de  Troyes,  etc. 
All  are  either  more  or  less  pure  chalk,  marl,  or  a  fairly 
white  clay,  pipeclay — which  is  also  used  for  making 
clay  pipes  and  for  removing  grease  spots. 


The  mineral  known  as  gypsum,  or  alabaster,  consists 
of  calcium  sulphate,  or  sulphate  of  lime,  its  composition 
being  expressed  by  CaSO4  +  2H./X  In  gypsum  the 


crystalline  structure  is  just  discernible,  whilst  other 
varieties,  such  as  the  so-called  "  marine  glass,"  occur 
in  considerable  quantities  as  large,  perfectly  transparent 
masses.  "  Russian  glass "  consists  of  large,  trans- 
parent lumps  possessing  the  specific  property  of 
gypsum,  viz.  that  of  cleaving  in  two  directions,  in  a 
high  degree.  Alabaster  is  composed  of  finely  granular 
masses,  which  are  either  quite  white,  or  else  yellowish, 
or  traversed  by  grey  veins.  This  variety  of  gypsum  is 
very  abundant  in  central  Italy,  and  the  best  blocks 
are  employed  for  the  production  of  works  of  art. 

Ordinary  gypsum,  which  frequently  occurs  in  the 
vicinity  of  dolomitic  limestones,  is  found  in  a  great 
variety  of  colours,  bluish-grey,  yellowish  or  reddish 
tints  being  the  most  common.  Pure  white  lumps, 
which  are  plentiful  in  some  deposits,  can  be  used  as 
white  pigment,  the  method  of  preparation  being  simple, 
viz.  merely  reducing  the  mass  to  powder.  This  is 
easily  effected,  the  specific  hardness  of  gypsum  being 
only  2 ;  and  in  many  cases  it  is  soft  enough  to  scratch 
with  the  finger-nail. 

If  the  original  gypsum  is  white,  the  powder  forms  a 
dazzling  white  flour  which,  notwithstanding,  is  of 
comparatively  little  value  as  a  pigment,  on  account  of 
its  low  covering  power.  For  this  reason,  powdered 
gypsum  is  chiefly  used  for  making  plaster  of  Paris 
(calcined  gypsum)  for  plaster  casts  and  stucco.  Gypsum 
may  also  be  employed  to  advantage  for  lightening 
various  colours,  since  it  is  inert  towards  even  the  most 


Large  areas  of  the  earth's  surface  are  covered  with 
clay,  which  often  attains  a  considerable  thickness. 


Nevertheless,  the  kind  of  clay  that  is  suitable  for  use 
as  pigment  is  comparatively  scarce.  The  principal 
requirement  for  this  purpose  is  a  pure  white  colour, 
but  by  far  the  great  majority  of  clays  are  either  yellow 
or  of  a  shade  between  blue  and  grey  (for  example  the 
clay  of  the  Vienna  basin). 

The  character  of  clay  is  just  as  varied  as  its  colour. 
In  some  places,  large  deposits  of  extremely  fine  clay 
are  found,  the  material,  when  mixed  with  water, 
forming  a  highly  plastic  mass  which,  when  dried  and 
subjected  to  slight  pressure,  furnishes  a  very  soft 
powder.  On  the  other  hand,  some  clays  are  so  inter- 
spersed with  large  quantities  of  sand,  large  stones  and 
the  debris  of  mussels,  that  they  cannot  be  used  until 
they  have  been  put  through  very  careful  mechanical 

This  great  divergence  in  the  physical  character  of 
clays  is  due  to  their  method  of  formation.  Clay 
originated  in  the  weathering  of  felspar,  which  chiefly 
consists  of  a  double  salt,  a  compound  of  the  silicates 
of  alumina  and  potash.  Under  the  influence  of  air 
and  water,  this  compound  is  decomposed,  the  potassium 
silicate  passing  into  solution,  whilst  the  aluminium 
silicate,  being  insoluble  in  water,  is  carried  away  by 
that  medium.  When  the  water  can  no  longer  carry 
the  particles  of  aluminium  silicate  in  suspension — for 
example  when  it  reaches  a  sea  or  lake — the  silicate 
settles  down  to  the  bottom,  and  a  deposit  of  clay  is 

If  the  original  felspar  was  very  pure,  and  in  particular 
very  low  in  iron,  the  resulting  clay  will  be  of  a  handsome 
white  colour.  An  example  of  this  is  afforded  by 
kaolin,  or  porcelain  earth,  which  is  preferably  used  for 
making  china.  If,  however,  the  felspar  contained  a 


considerable  proportion  of  ferric  oxide,  the  resulting 
clay  is  yellow;  and  if  stones  or  mussel  shells  became 
incorporated  with  the  clay  prior  to  deposition,  these 
bodies  will  be  found  as  inclusions  in  the  deposit,  and 
such  clay  will  require  much  troublesome  preparation- 
grinding  and  levigation — before  it  is  fit  for  use. 

For  the  purposes  of  the  colour-maker,  the  most 
suitable  clay  is  one  that  is  pure  white,  free  from  inclu- 
sions, and  does  not  change  colour  when  exposed,  in  a 
finely  divided  state,  to  the  action  of  the  air.  Many 
clays  that  were  originally  white  gradually  assume  a 
yellow  tinge  on  prolonged  exposure  to  air  and  moisture, 
because  the  clay  contained  ferrous  oxide,  which 
changes,  in  the  air,  to  the  stronger  pigment,  ferric 

Many  kinds  of  clay  merely  require  a  simple  levigation 
to  fit  them  for  use  as  pigment.  The  lumps  of  freshly 
dug  clay  are  placed  in  large  tanks,  etc.,  filled  with  water 
and  stirred  up  continuously  in  order  that,  instead  of 
forming  a  plastic  mass  which  is  very  difficult  to  dis- 
tribute in  water,  the  particles  detached  from  the  lumps 
may  pass  at  once  into  suspension.  This  turbid  water 
is  then  transferred  to  another  tank,  etc.,  where  the 
minute  particles  of  clay  are  allowed  to  settle  down,  and 
the  water  becomes  quite  clear. 

Where  this  work  is  carried  on  on  a  large  scale,  it  is 
advisable  to  put  the  freshly  won  clay  into  large  pits 
close  to  the  clay  deposit,  and  to  leave  it  there,  covered 
with  water,  during  the  winter  season.  The  freezing 
of  the  water  breaks  down  the  larger  lumps  of  clay, 
by  the  resulting  expansion,  and  this  facilitates  the 
subsequent  levigation,  the  cohesion  between  the 
particles  being  destroyed. 

If  the  clay  contains  larger  proportions  of  lime  or 


magnesia,  a  little  experience  will  enable  their  presence 
to  be  detected  at  once  by  the  way  the  clay  behaves 
on  being  placed  in  contact  with  water.  Pure  clay 
quickly  forms  a  fatty  and  extremely  plastic  paste,  and 
sticks  closely  to  the  tongue  when  applied  in  the  dry 
state.  On  the  other  hand,  clay  containing  much  lime 
or  magnesia  is  far  less  plastic  when  mixed  with  water, 
and  the  dry  clay  hardly  adheres  to  the  tongue  at  all. 

These  latter  clays  are  classed  as  poor  or  lean,  in 
contrast  to  the  fat,  plastic  kinds.  For  certain  purposes 
for  which  clay  is  used  as  pigment,  these  admixtures 
are  not  harmful ;  whereas  others,  especially  quartz 
sand  and  mica,  not  infrequently  present  in  white  clays, 
constitute  a  serious  drawback. 

As  already  mentioned,  clay  is  formed  by  the  weather- 
ing of  felspar,  which  is  a  constituent  of  granite  and 
gneiss,  both  rocks  composed  of  quartz,  mica  and  felspar. 
When  the  clay  has  been  derived  from  the  weathering 
of  such  rocks,  it  is  easy  to  understand  that  it  may 
contain  admixtures  of  quartz  and  mica,  which  are 
frequently  visible  to  the  naked  eye,  or  at  any  rate 
under  the  microscope.  Whereas  clay  forms  a  white, 
amorphous  mass,  the  grains  of  quartz  sand  are  decidedly 
crystalline,  transparent  and  of  vitreous  lustre;  the 
scales  of  mica,  on  the  other  hand,  appearing  as  thin 
tabular  crystals,  mostly  of  a  green  or  brown  colour 
and  exhibiting,  when  viewed  at  certain  angles,  a 
brilliant  metallic  sheen. 

Quartz  sand  can  be  eliminated  from  clay  witnout 
any  special  difficulty,  quartz  being  of  higher  specific 
gravity  and  therefore  settling  down  quickly,  leaving 
the  delicate  particles  of  clay  in  suspension  in  the  liquid. 
The  scales  of  mica  are  harder  to  get  rid  of,  their  tabular 
form  retarding  deposition  from  the  suspending  liquid ; 


and  on  this  account,  several  washings  are  often  required 
to  separate  them  completely. 

In  all  cases  where  clay  is  to  be  used  as  a  white 
distemper,  the  presence  or  absence  of  lime  is  immaterial ; 
but  where  it  is  to  be  employed  for  removing  grease, 
lime  is  a  drawback.  This  is  also  sometimes  the  case 
when  the  clay  is  wanted  for  the  purposes  of  the  colour 
manufacturer.  The  author  has  found,  by  experience, 
that  perfectly  pure,  white  clay  forms  a  good  paint, 
in  a  vehicle  of  oil  or  varnish — a  purpose  to  which  it 
has,  so  far,  been  seldom  applied,  if  at  all.  Such  paint 
is  of  good  covering  power,  and  possesses  the  valuable 
property  of  remaining  quite  unaffected  by  atmospheric 

If,  however,  the  clay  contains  even  but  a  small 
quantity  of  lime,  it  cannot  possibly  be  used  as  an  oil 
or  varnish  paint,  for  though  the  freshly  made  paint 
has  a  very  good  appearance,  its  character  soon  changes, 
turning  viscous  and  suffering  a  considerable  diminution 
of  covering  power.  Thinning  with  turps  or  boiled 
oil  results  in  the  formation  of  small  lumps,  so  that  it 
is  quite  impossible  to  obtain  a  uniform  coating  on  even 
a  small  surface. 

This  behaviour  is  apparently  due  to  the  presence  of 
the  lime,  the  explanation  being  that  the  fatty  acids 
always  present  in  the  oils  and  varnishes  used  for  the 
paint  combine  with  the  lime  to  form  compounds 
which,  from  the  standpoint  of  the  chemist,  must  be 
regarded  as  soaps.  The  small  lumps  already  mentioned 
really  consist  of  lime  soap,  and  the  formation  of  these 
colourless  compounds  accounts  for  the  lessened 
covering  power. 

Given  a  fine  white  clay,  otherwise  capable  of  forming 
a  valuable  pigment,  it  is  sometimes  possible,  by  simple 


means,  to  eliminate  accompanying  lime,  provided 
the  amount  of  the  latter  is  not  too  great,  and  also 
provided  that  very  cheap  hydrochloric  or  acetic  acid 
is  available.  The  acid  need  not  be  pure,  and  the  impure 
but  very  strong  pyroligneous  acid,  which  is  very 
cheap  on  account  of  its  empyreumatic  smell,  may  be 

To  eliminate  lime  from  the  clay,  the  still  moist 
levigated  mass  is  introduced,  in  small  quantities,  into 
a  vat  containing  the  requisite  quantity  (see  later)  of 
hydrochloric  or  acetic  acid,  the  addition  being  con- 
tinued until  the  liquid  gives  only  a  faintly  acid  reaction 
with  blue  litmus  paper.  When  the  clay  is  run  in, 
effervescence  is  produced  by  the  liberation  of  the 
carbon  dioxide  displaced  by  the  stronger  acid  employed. 

The  amount  of  lime  present  in  a  clay  may  be  deter- 
mined by  very  simple  means.  A  small  sample  of  the 
clay  is  dried  by  artificial  heat,  until  of  constant  weight, 
and  exactly  100  parts  by  weight  of  the  dry  mass  are 
placed  in  a  glass  and  suffused  with  hydrochloric  acid, 
sufficient  of  the  latter  being  used  to  make  the  liquid 
still  strongly  acid  after  effervescence  has  ceased. 

The  contents  of  the  glass  are  transferred  to  a 
filter,  and  washed  with  pure  water  so  long  as  the 
washings  continue  to  redden  blue  litmus  paper. 
The  residue  is  then  dried  until  of  constant  weight,  and 
the  difference  between  the  initial  and  final  weights 
will  give  the  percentage  of  substances  soluble  in 
hydrochloric  acid. 

After  performing  this  simple  test  on  a  clay,  it  is 
easy  to  calculate  the  quantity  of  acid  needed  to  extract 
all  the  soluble  constituents  from  a  given  weight  of 
the  material.  All  that  is  necessary  is  to  measure 
the  volume  of  acid  required  to  extract  a  small  quantity 


of  the  clay  completely.  Thus,  if  one  pint  of  the  acid 
at  disposal  is  sufficient  to  treat  one  pound  of  the  clay, 
the  amount  needed  for  a  given  quantity  of  clay  is  a 
simple  matter  of  calculation. 

Since,  on  account  of  the  cost  of  pure  hydrochloric 
acid,  crude  acid  will  always  be  used,  it  will  be  necessary 
to  remember  that  this  crude  acid  always  contains 
ferric  oxide  in  solution — this  being  the  cause  of  its 
yellow  colour.  If  the  amount  of  acid  taken  is  barely 
sufficient  to  combine  the  whole  of  the  lime,  leaving 
the  latter  slightly  in  excess,  the  ferric  oxide — which 
would  otherwise  tinge  the  clay  yellow — will  be 

If,  on  the  other  hand,  the  acid  is  in  excess,  the 
clay  is  obtained  free  from  all  constituents  soluble  in 
the  acid.  The  purified  clay  must  then  be  freed  from 
the  calcium  chloride,  formed  by  dissolving  the  lime,  by 
a  thorough  washing,  since  the  clay  would  otherwise 
always  remain  moist  on  account  of  the  hygroscopic 
properties  of  the  chloride  in  question.  Moreover,  any 
small  residuum  of  free  acid  would  constitute  a  draw- 
back on  the  clay  being  mixed  with  other  colours. 

Calcium  chloride  is  very  soluble  in  water,  and  there- 
fore can  be  completely  removed  from  the  clay  by 
washing.  The  purified  clay  is  left  to  settle  down  as 
completely  as  possible,  and  after  drawing  the  liquid  off 
from  the  sediment,  the  latter  is  suffused  with  pure 
water  and  left  to  settle  once  more.  As  a  rule,  two  such 
washings  will  cleanse  the  clay  of  calcium  chloride  and 
free  acid  sufficiently  to  render  the  product  suitable 
for  any  purpose. 

When  large  quantities  of  clay  have  to  be  treated 
in  this  manner,  considerable  amounts  of  calcium 
chloride  solution  will  be  obtained,  which  can  be  advan- 


tageously  utilised  for  the  production  of  precipitated 
chalk,  all  that  is  necessary  being  to  collect  the  liquor 
in  a  large  tank  and  treat  it  with  a  small  quantity  of 
slaked  lime,  to  transform  the  surplus  free  acid  into 
calcium  chloride  and  precipitate  the  ferric  oxide  present 
in  solution.  At  the  end  of  a  few  days  the  liquor  in 
the  tank  will  consist  of  a  very  pure  solution  of  calcium 
chloride  which  will  furnish  an  excellent  precipitated 
chalk  when  treated  in  the  manner  already  described 
under  that  heading. 


This  mineral — chemically,  barium  sulphate,  BaSO4 — 
occurs  native,  as  extensive  deposits,  in  many  places — 
England,  Bohemia,  Saxony,  Styria,  etc.  It  sometimes 
forms  handsome  tabular  crystals,  but  more  frequently 
compact  masses,  which  may  be  pure  white,  grey  yellow, 
etc.,  in  colour,  and  are  distinguished  by  high  specific 
gravity  (usually  4-3-4-7),  to  which  the  mineral  owes  its 
name.  This  high  density  also  limits  the  application 
of  the  mineral,  and  it  cannot  be  used  as  a  pigment, 
in  the  true  sense  of  the  term,  being  only  suitable  as  an 
adjunct  to  artificially  prepared  colours. 

The  employment  of  barytes  in  the  colour  industry 
is  often  regarded  as  adulteration,  which,  however,  it 
is  not  when  the  case  is  considered  from  the  right  point 
of  view.  For  instance,  the  only  preparation  which  can 
properly  be  termed  white  lead  consists  of  basic  lead 
carbonate.  This,  when  pure,  is  a  rather  expensive 
pigment,  whereas,  for  certain  purposes,  the  consumer 
requires  a  product  that  can  be  obtained  at  a  low  price. 
In  order  to  satisfy  this  demand,  the  only  course  open 
to  the  colour-maker  is  to  mix  the  white  lead  with  a 


cheap  white  substance,  which  enables  him  to  turn  out 
different  grades  of  white  lead,  which,  although  low  in 
price,  are  far  inferior  to  the  pure  article  in  covering 
power.  Pure  white  lead  being  itself  a  very  heavy 
substance,  the  only  bodies  suitable  as  adjuncts  are 
such  as  are  also  of  high  specific  gravity;  and  of  all 
the  cheap  pigments  known,  heavy  spar  is  the  only 
one  endowed  with  this  property.  Consequently,  this 
substance  is  extensively  used  in  making  the  cheaper 
grades  of  white  lead  and  the  pale  kinds  of  chrome 

The  only  cases  in  which  the  addition  of  heavy  spar 
to  a  colour  can  be  regarded  as  an  intentional  fraud  on 
the  consumer  is  when  he  is  sold,  as  pure  white  lead, 
chrome  yellow,  etc.,  a  product  really  composed  of  a 
mixture  of  such  colour  and  barytes.  Moreover,  the 
presence  of  barytes  in  white  lead  can  be  easily  detected 
by  a  simple  examination,  pure  white  lead  readily 
dissolving,  with  considerable  effervescence,  in  strong 
nitric  or  acetic  acid,  whereas  barytes  is  insoluble  in 
all  acids,  and  therefore  remains,  as  a  heavy  white 
powder,  at  the  bottom  of  the  vessel.  In  this  way 
both  the  presence  and  amount  of  barytes  contained 
in  a  sample  of  white  lead  or  chrome  yellow  can  easily 
be  ascertained. 

The  preparation  of  barytes  for  the  purposes  of  the 
colour-maker  is  entirely  a  mechanical  operation.  The 
barytes,  which  though  fairly  hard  is  easily  reduced, 
is  crushed  with  stamps,  ground  in  a  mill  and  finally 
levigated,  it  being  impossible  to  obtain  a  sufficiently 
fine  powder  even  by  repeated  grinding. 

Native  barytes  must  not  be  confounded  with  the 
artificial  barium  sulphate  sold  as  permanent  white 
or  blanc  fixe,  which  is  an  extremely  finely  divided 


barium  sulphate  obtained  by  precipitating  a  solution  of 
a  barium  salt  with  sulphuric  acid  or  a  soluble  sulphate, 
and  is  a  painters'  colour  that  is  highly  prized  for  certain 
purposes.  Both  the  native  sulphate  and  the  artificial 
variety  have  the  property  of  remaining  completely 
unaltered  by  exposure  to  air,  and  they  can  therefore 
be  mixed  with  any  kind  of  pigment  without  fear  of 
the  colour  deteriorating. 

As  a  rule,  barytes  is  first  roughly  crushed  in  edge- 
runner  mills  or  stamps,  and  then  ground  to  the  extreme 
degree  of  fineness  obtainable  in  ordinary  mills.  Even 
with  the  greatest  care,  however,  it  is  impossible  by  this 
means  to  obtain  sufficient  fineness  of  division  for  mixing 
with  fine  colours,  the  only  way  in  which  this  can  be 
accomplished  being  by  levigation. 

Given  a  fairly  pure  white  barytes  to  begin  with, 
levigation  furnishes  a  handsome  white  pigment  that 
can  be  mixed  with  colours  of  any  kind;  but  when 
used  by  itself  in  association  with  oil  or  varnish,  its  . 
covering  power  is  very  low  and  the  colour  never 
perfectly  white.  Native  barytes  is  therefore  unsuitable, 
as  such,  for  paint. 

Varieties  that  are  not  pure  white  are  sometimes 
corrected  with  ultramarine,  added  in  the  grinding- 
mill.  If  the  yellow  tinge  is  due  to  iron  compounds, 
this  can  often  be  remedied  by  treating  the  finely  ground 
material  with  hydrochloric  acid,  which  dissolves 
them  out,  this  treatment  being  followed  by  a  thorough 
washing  with  pure  water. 

As  already  mentioned,  white  lead  is  most  frequently 
mixed  with  barytes,  this  being  usually  added  when 
the  white  lead  is  being  ground,  by  feeding  the  two 
materials  to  the  mill  and  grinding  them  together. 

The  crudeness  of  mechanical  methods  of  reduction 


is  clearly  exemplified  by  comparing  the  most  carefully 
ground  and  levigated  barytes  with  that  obtained 
by  artificial  means.  The  permanent  white  largely 
used  in  the  production  of  wall-paper,  and  quite  unalter- 
able in  air,  is,  chemically  speaking,  identical  with 
native  barytes,  viz.  barium  sulphate.  The  two  also 
seem  to  be  identical  in  crystalline  habit,  as  is  usual 
in  the  case  of  one  and  the  same  mineral,  whether  native 
or  prepared  by  artificial  means.  Artificial  barytes 
is  obtained  by  treating  a  soluble  salt  of  barium  with 
sulphuric  acid,  or  a  solution  of  sodium  sulphate  (Glauber 
salt),  so  long  as  a  precipitate  continues  to  form. 

This  precipitate  is  barium  sulphate,  which  subsides 
completely  on  account  of  its  extreme  insolubility,  this 
being  greater  than  that  of  any  other  salt  known.  The 
rapid  rate  of  deposition  results  in  the  formation  of 
extremely  small  crystals,  which,  being  colourless  and 
reflecting  the  light  completely,  appear  to  be  perfectly 
white.  Even  when  permanent  white  is  applied  in 
very  thin  layers  to  any  surface,  its  covering  power  is 
very  considerable,  by  reason  of  the  extremely  fine  sub- 
division of  the  material. 

This  behaviour  of  artificial  barytes  in  comparison 
with  that  of  the  natural  product,  affords  an  important 
hint  in  connection  with  the  preparation  of  earth  colours, 
namely,  that  in  order  to  obtain  products  of  specially 
good  quality,  the  endeavour  should  be  to  reduce  the 
raw  materials  to  the  finest  condition  possible.  This 
result  is  accomplished  most  securely  by  bestowing 
the  greatest  care  on  grinding  and  levigation ;  and  it 
is  therefore  highly  important  that  the  manufacturer 
should  select,  from  the  various  apparatus  used  in 
reducing  the  materials,  those  that  are  best  adapted 
for  the  purpose. 



Although  carbonate  of  magnesia  is  seldom  used 
alone  as  a  pigment,  it  can  be  advantageously  employed 
as  such  when  circumstances  permit.  It  is  met  with 
not  infrequently,  in  Nature,  in  a  crystalline  form,  as 
magnesite  or  bitter  spar,  the  latter  name  arising  from 
the  fact  that  the  soluble  salts  of  magnesia  have  a  bitter 
taste.  Still  more  frequently,  magnesia  occurs  in 
association  with  calcium  carbonate,  in  the  mineral 
dolomite,  which  contains  up  to  20%  of  magnesia. 

A  less  abundant  native  mineral  is  hydromagnesite, 
which  consists  of  basic  magnesium  hydrocarbonate. 
Hydromagnesite  is  a  very  light,  chalk- white  mass, 
with  a  non-greasy  feel,  which,  when  reduced  to  a 
soft  powder,  forms  an  excellent  material  for  paint. 
It  is  highly  inert,  in  a  chemical  sense,  and  can  therefore 
be  mixed  with  the  most  delicate  colours,  having  no 
other  effect  thereon  than  to  render  them  lighter  in 

This  product  can  also  be  prepared  artificially,  by 
treating  a  dissolved  magnesium  salt  with  a  solution 
of  carbonate  of  soda,  the  result  being  the  formation 
of  a  pure  white  precipitate,  which  is  very  brilliant 
when  dry,  and  is  characterised  by  unusually  low  specific 
gravity.  In  some  places,  conditions  are  such  that 
this  preparation  can  be  made  on  a  large  scale  at  very 
low  cost.  For  instance,  there  is  a  spring  at  Bilin,  in 
Bohemia,  the  water  of  which  contains  large  quantities 
of  alkali  carbonates  in  solution ;  whilst  in  the  vicinity 
of  Saidschlitz  is  a  spring  fairly  rich  in  magnesia  salts. 
The  waters  from  these  two  springs  are  concentrated 
by  evaporation,  and  mixed  in  large  tanks ;  and  when  a 
sufficient  deposit  of  the  resulting  basic  carbonate  of 


magnesia  has  accumulated,  it  is  taken  out  of  the  tanks, 
placed  on  linen  niters  and  washed  with  water.  The 
residue  is  dried  slowly,  without  the  employment  of  a 
high  temperature,  and  then  forms  a  white  powder, 
which  is  very  light  and  can  be  used  for  a  number  of 
purposes,  chiefly  medicinal,  though  it  is  also  well 
adapted  as  a  material  for  paint. 

For  this  latter  purpose  it  is,  however,  far  too  expen- 
sive; but  since  the  conditions  obtaining  at  Bilin  are 
certain  to  occur  elsewhere,  we  have  included  carbonate 
of  magnesia  among  the  earth  colours. 

On  account  of  its  specific  lightness,  carbonate  of 
magnesia  is  specially  adapted  for  making  pale  shades 
of  certain  delicate  lake  colours,  which,  if  toned  with 
even  perfectly  pure  chalk,  would  undergo  alteration  in 
course  of  time.  Carmine,  for  instance,  can  be  graded, 
by  the  addition  of  carbonate  of  magnesia,  into  every 
possible  variety  of  shades  between  the  pure  red  of 
carmine  itself  and  the  palest  pink;  and  the  resulting 
colours  are  quite  permanent  whether  mixed  with  gum 
solution  or  any  other  vehicle. 


Although  this  mineral  is  not  used  as  a  pigment  by 
itself,  it  must  be  mentioned  here  because  it  is  not 
infrequently  employed  for  mixing  with  other  colours, 
and  is  also  used  in  the  wall-paper  industry.  It  also 
serves  to  distribute  certain  pigments  in  a  state  of  fine 
division,  the  "  rouge  vegetal  "  of  the  perfumer,  for 
example,  usually  consisting  of  talc  and  a  small  quantity 
of  very  fine  carmine. 

In  commerce  the  name  talc  is  sometimes  applied 
to  two  separate  minerals,  true  talc  and  steatite  or  soap- 


stone.  The  former  is  rarely  met  with  native  as  well- 
defined  crystals,  mostly  occurring  as  scaly  masses  in 
primitive  rocks.  Thin  pieces  exhibit  a  certain  degree 
of  flexibility.  The  hardness  of  this  mineral  is  so  small 
that  it  can  be  scratched  with  the  finger-nail;  and  its 
sp.  gr.  is  2'9-2-8.  Talc  is  easily  scraped,  and  the 
powder  remains  sticking  to  the  knife,  a  property  which 
renders  the  substance  difficult  to  reduce  to  powder, 
because  it  balls  together  and  takes  a  very  long  time  to 
convert  into  a  fine  flour.  The  process  is  facilitated  by 
calcining  the  talc  and  quenching  it  in  cold  water,  this 
treatment  increasing  the  hardness  and  at  the  same  time 
making  it  more  brittle,  and  thus  more  easy  to  pulverise. 
A  characteristic  feature  of  all  the  talc  minerals  is 
their  peculiar  greasy  appearance  and  feel.  The  colour 
varies,  white  pieces  alone  being  of  any  use  to  the  colour 
manufacturer.  The  yellow-  or  green-tinged  varieties 
owe  their  shade  to  the  presence  of  ferric  and  ferrous 
oxides.  In  chemical  composition,  talc  consists  of  a 
combination  of  magnesium  sillicate  with  hydrated 
silica,  the  supposed  formula  being  :  4MgO  .  SiO2  + 
H2O  .  SiO2,  and  the  percentage  composition  :  silica, 
62-6%  ;  magnesia,  32-9%  ;  water,  4-9%. 


Steatite  so  closely  resembles  talc  in  most  of  its 
properties,  that  the  two  minerals  were  long  regarded 
as  identical.  Whereas,  however,  talc  is  scarcely  acted 
upon  at  all  by  the  strongest  acids,  steatite  is  completely 
decomposed  by  prolonged  boiling  therewith,  although 
both  minerals  have  exactly  the  same  composition. 

As  a  pigment,  steatite  is  far  more  important  than 
talc,  and,  as  French  chalk,  is  largely  used  for  drawing 


or  writing.  To  prepare  it  for  this  purpose  pure  white 
steatite  requires  no  preliminary  treatment,  beyond 
cutting  the  large  lumps  up  into  quadrangular  prisms, 
which  are  mounted  in  wood,  like  lead  pencil,  and  used 
for  writing  on  the  blackboard.  The  powder  produced 
in  the  cutting  process  is  made  up  into  pastel  crayons. 
With  this  object,  the  powder  is  mixed  with  a  sufficient 
quantity  of  some  mineral  pigment  to  produce  a  mass 
of  the  desired  shade,  and  is  kneaded  to  a  stiff  paste  with 
water  containing  an  adhesive  such  as  gum,  glue  or 
tragacanth  mucilage.  The  mass  is  shaped  into  prisms, 
which,  when  dry,  are  cut  into  pencils  and  mounted 
in  wood.  Steatite  being  like  talc,  without  action  on 
even  the  most  delicate  colours,  can  be  used  as  a  diluent 
in  the  preparation  of  light  shades. 



ALL  the  yellow  earth  colours,  without  exception, 
have  ferric  oxide  as  their  colouring  principle,  the 
differences  in  shade  being  entirely  due  to  the  varying 
proportion  in  which  that  oxide  is  present.  The  various 
names  under  which  they  are  known  date  back  to  a 
period  when  the  chemical  nature  of  these  colours  was 
still  unknown,  and  have  been  mostly  derived  from  the 
locality  of  origin. 

'file  yellow  earths  can  therefore  be  divided  into  two 
groups,  according  to  their  chemical  character.  The 
first  group,  in  which  the  ferric  oxide  is  present  as 
hydroxide,  comprises  all  the  ochres,  Siena  earth,  and 
a  number  of  others  which  are  obtained  from  native 
ochre  by  special  treatment.  In  the  colours  of  the 
second  group,  ferric  oxide  is  still  the  colouring  principle, 
but  is  combined  with  other  substances  in  place  of  water. 

It  is,  as  a  matter  of  fact,  incorrect  to  rank  the  ochres 
in  general  as  yellow  earths,  because  they  can  be  made 
to  yield  nearly  every  variety  of  colour  from  the  palest 
yellow  to  the  deepest  red,  brown  and  violet.  These 
colours  merit  the  particular  attention  of  the  colour- 
maker  and  the  painter,  being  distinguished  by  very 
low  cost  of  production,  unusual  permanence  and  beauty 
of  tone.  In  the  interests  of  that  highly  important 
matter  to  the  artist,  namely  the  production  of  colours 



of  unlimited  permanence,  it  is  desirable  that  colour 
manufacturers  should  bestow  greater  care  on  the 
manufacture  of  these  colours  than  has  hitherto  been 
the  case.  An  extremely  favourable  point  about  nearly 
all  these  pigments  is  that  they  can  be  very  cheaply 
prepared  by  artificial  means,  so  that  the  manufacturer 
is  in  a  position  to  turn  out  a  large  number  of  the  hand- 
somest and  most  durable  colours  with  a  small  amount 
of  expense  and  labour. 


Ochres  are  found  in  many  localities,  most  frequently 
in  stratified  rock  and  rubble.  The  deposits  are  rarely 
extensive,  mostly  occurring  in  pockets  or  beds.  Where- 
ever  found,  ochre  may  be  termed  a  secondary  product, 
that  is  to  say,  one  that  has  been  formed  through  the 
destruction  of  other  minerals.  The  analysis  of  ochres 
from  different  deposits  shows  great  divergence  in 
composition ;  and  some  consist  almost  entirely  of  pure 
ferric  hydroxide,  that  has  already  undergone  natural 
levigation  and  can  be  used  as  a  pigment  as  soon  as  dug. 

Such  a  form  is,  however,  rare,  and  most  ochres  are 
intermixed  with  smaller  or  larger  amounts  of  extraneous 
minerals,  the  contamination  being  sometimes  so  great 
as  to  preclude  the  use  of  the  ochre  as  pigment  by  reason 
of  the  high  outlay  required  for  extracting  the  colouring 

Occasionally,  the  ferric  hydroxide  is  associated  with 
a  certain  proportion  of  clay,  and  as  this  increases,  the 
ochre  passes  over  into  ferruginous  clay.  This  class 
can  also  be  used  as  pigment,  in  certain  circumstances, 
that  is  to  say  when  it  is  sufficiently  rich  in  ferric  oxide 
to  furnish  a  deep  red  mass  on  calcination.  When, 



however,  the  proportion  of  ferric  oxide  is  low,  its 
pigmentary  power  is  no  longer  sufficient,  and  the  clay 
has  not  the  requisite  beauty  of  colour.  The  ordinary 
earth  used  for  making  tiles  is  an  example  of  this  class, 
its  colour  in  the  raw  state  being  an  ugly  brownish- 
yellow,  but  turning  a  dull  "  brick  "  red  when  fired. 

In  some  deposits  the  ferric  oxide  is  accompanied  by 
lime.  Unless  the  latter  exceeds  a  certain  proportion, 
such  ochres,  too,  are  suitable  as  pigments,  the  lime 
being  easily  removed  by  simple  levigation ;  but  when 
the  amount  of  lime  is  high,  it  is  difficult  to  obtain 
certain  highly  coloured  shades  of  ochre  from  such 
material.  These  shades  entail  the  calcination  of  the 
ochre,  and  the  temperature  required  is  oftentimes 
insufficient  to  transform  the  lime  into  the  caustic  state. 
Moreover,  the  presence  of  caustic  lime  would  be  a 
drawback  in  some  cases,  it  being  then  impossible  to 
mix  the  ochre  with  other  colours  without  endangering 
the  shade  through  the  action  of  the  lime  on  these  latter. 

The  following  analyses  will  show  the  percentage 
composition  of  ochres  from  various  deposits  : 

Ochre  from — 



St.  Georges. 

Ferric  oxide 




Lime         .... 


Alumina    .... 


Magnesia  .... 
Silica         .... 




Water        .          . 




In  the  majority  of  cases  the  mineralogical  character- 
istics of  an  ochre  enable  conclusions  to  be  formed  as 
to  its  suitability  as  pigment.     Good  ochre  is  more  or 


less  yellow  to  dark  brown  in  colour,  and  can  easily  be 
crushed  between  the  fingers  to  a  soft,  fine  powder 
which  feels  like  powdered  steatite  and  does  not  produce 
a  sensation  of  grittiness,  this  latter  indicating  the 
presence  of  fine  grains  of  sand  in  the  ferric  oxide.  The 
behaviour  of  the  ochre  in  presence  of  water  is  specially 
important.  If  it  adheres  firmly  to  the  tongue,  and 
forms  a  fairly  plastic  paste  when  mixed  with  a  little 
water,  the  mineral  contains  a  large  percentage  of  ferric 
oxide,  and  as  a  rule  will  yield  ochre  of  good  colour. 

In  general  it  may  be  said  that  the  value  of  an  ochre 
varies  directly  with  its  content  of  ferric  hydroxide  or 
oxide,  because  when  this  is  large  the  ochre  will  furnish 
a  wide  range  of  colours  under  suitable  treatment. 

A  simple  test  for  quality  consists  in  weighing  out 
an  exact  small  quantity  (10  grms.),  and  heating  it  to 
a  temperature  not  exceeding  110°  C.,  until  the  weight 
remains  constant.  A  simple  calculation  then  gives  the 
amount  of  uncombined  water  in  the  sample.  Since 
the  proportion  of  such  water  varies  in  different  parts 
of  one  and  the  same  deposit,  the  test  must  be  repeated, 
in  order  to  obtain  accurate  results,  on  samples  taken 
from  different  points,  or,  preferably,  on  a  properly 
prepared  average  sample. 

Even  drying  changes  the  colour  of  ochre  considerably. 
To  ascertain  the  behaviour  of  an  ochre  on  calcination, 
a  large  sample  is  dried  at  110°  C.  until  the  weight  is 
constant,  and  divided  up  into  a  number  of  small  samples 
weighing,  say,  10  grms.  each.  The  samples  are  then 
heated  to  different  temperatures,  one  to  the  melting- 
point  of  lead,  another  to  that  of  zinc,  and  so  on. 

The  higher  the  temperature  employed,  the  more  will 
the  colour  of  the  ochre  approximate  to  red ;  and 
specimens  very  rich  in  ferric  oxide  will  give  bright  red 


colours.  Beyond  this  range,  a  further  increase  in 
temperature  will  give  violet  shades,  varying  with 
the  temperature  and  the  duration  of  heating.  After 
this  preliminary  test,  it  is  desirable  to  make  another 
on  a  larger  scale,  with  quantities  up  to  about  i  Ib. 
For  this  test,  the  different  kinds  of  ochre  frequently 
found  in  the  same  deposit  should  be  mixed  together, 
in  order  to  obtain  an  idea  of  what  the  mean  product, 
obtained  in  working  on  the  large  scale,  will  be  like. 

On  the  whole,  the  results  of  this  second  test  will  be 
the  same  as  in  the  first  series,  the  only  object  of  the 
second  test  being  to  gain  information  which  may  be 
particularly  valuable  in  practical  work.  The  bottles 
in  which  the  calcined  samples  are  stored  should  be 
marked  with  the  temperature  and  length  of  heating,  so 
that,  when  it  is  subsequently  desired  to  obtain  an  ochre 
corresponding  to  a  particular  sample,  all  that  is 
necessary  will  be  to  heat  it  to  the  same  degree  from 
the  same  length  of  time.  The  performance  of  this 
simple  test  will  be  of  great  assistance  in  standardising 
the  work  with  a  minimum  loss  of  time. 

When  it  is  desired  to  ascertain  the  composition  of 
an  ochre  superficially  its  behaviour  towards  hydro- 
chloric acid  maybe  noted.  A  weighed  quantity  of  the 
freshly  dug  (undried)  ochre  is  treated  with  pure  acid, 
free  from  iron,  which  will  dissolve  out  the  ferric  oxide 
and  lime,  leaving  clay  and  quartz  sand  behind.  The 
presence  of  lime  is  indicated  by  effervescence  on  contact 
with  the  acid ;  and  if  there  is  no  effervescence,  lime  is 
absent.  At  the  end  of  several  hours  the  acid  is  care- 
fully decanted  from  the  undissolved  residue  which  is 
then  stirred  up  with  water,  left  to  subside,  and  weighed 
when  dry.  This  method  will  give  the  amount  of 
substances,  other  than  ferric  oxide  and  lime,  in  the 


sample.  These  substances  usually  consist  of  clay  or 

For  a  quantitative  determination,  a  small  quantity 
—usually  I  grm. — is  weighed  out,  treated  with  a 
corresponding  amount  of  hydrochloric  acid,  and  the 
solution  filtered  into  a  glass.  The  residue  on  the  filter 
is  washed  with  distilled  water,  the  washings  being 
united  to  the  acid  solution. 

This  solution  is  treated  with  ammonia  so  long  as  a 
precipitate  of  ferric  hydroxide  continues  to  form,  this 
being  collected  on  a  tared  filter  and  dried  at  110°  C. 
The  precipitate  may  be  regarded  as  pure  ferric 
hydroxide,  and  its  weight  will  indicate  the  proportion 
of  hydroxide  in  the  ochre  with  sufficient  accuracy 
for  technical  purposes. 

In  reality,  however,  it  is  not  pure  ferric  hydroxide, 
but  contains  in  addition  all  the  oxides  that  are  precipi- 
table  by  ammonia,  lime  being  always  carried  down  as 
well.  It  is  therefore  desirable  to  dissolve  the  precipitate 
with  a  little  hydrochloric  acid,  and  reprecipitate  with 


In  many  places  ochre  is  only  put  through  a  very 
simple  mechanical  preparation  before  being  sold  for 
pigment,  namely  left  to  dry  in  the  air  so  that  most  of 
the  uncombined  water  evaporates.  No  matter  how 
this  drying  process  is  protracted,  however,  it  is  impos- 
sible to  get  rid  of  all  the  water  in  this  way,  a  certain 
proportion  being  retained  by  the  hygroscopic  action 
of  the  ferric  hydroxide,  and  to  expel  this  the  mass 
must  be  heated  to  above  100°  C.  Drying  is  usually 
succeeded  by  pulverising  and  sifting  the  loose  earthy 
mass,  which  is  then  ready  for  sale. 


When  the  ochre  contains  sand  or  stones,  this  treat- 
ment is  not  sufficient,  and  levigation  is  necessary.  No 
particular  trouble  is  involved,  the  mineral  being  fairly 
heavy  as  the  result  of  its  content  of  ferric  hydroxide. 
A  simple  method  of  treatment  suffices  to  improve  the 
value  of  the  ochre  considerably,  and  enables  a  grade 
that  is  not  particularly  bright-coloured  in  its  natural 
condition  to  be  converted  into  products  of  very  hand- 
some tone  and  various  shades.  This  treatment  con- 
sists in  heating  the  raw  ochre  to  a  definite  temperature, 
during  which  process  the  colour  changes  progressively, 
and  any  desired  tone  can  be  obtained  by  suddenly 
cooling  the  hot  mass. 

The  reason  for  this  phenomenon  is  that  the  higher 
the  temperature,  the  larger  the  amount  of  water  driven 
off  from  the  ferric  hydroxide,  until  finally,  when  a  very 
high  temperature  has  been  reached,  the  whole  of  the 
water  is  expelled,  and  the  ferric  hydroxide  is  trans- 
formed into  ferric  oxide.  The  hydroxide  is  brown, 
whereas  the  oxide,  provided  the  temperature  has  not 
been  raised  too  high,  exhibits  the  characteristic  colour 
known  as  "  iron  red." 

Consequently,  the  colour  of  moderately  calcined 
ochre  ranges  through  a  whole  scale  frcm  brown  to  red ; 
and  the  higher  the  temperature  employed,  the  redder 
the  tone.  If  the  heating  be  protracted  after  all  the 
hydroxide  has  become  oxide,  the  latter  undergoes 
molecular  change,  increasing  considerably  in  density 
and  altering  in  colour;  and  after  very  prolonged 
heating,  the  colour  finally  becomes  violet. 

The  calcination,  or  burning,  of  ochre  is  ordinarily 
performed  in  a  very  crude  manner.  The  mineral  is 
crushed  to  the  size  of  peas,  and  spread  out  on  an  iron 
plate  which  is  made  red-hot.  As  soon  as  the  ochre 


has  reached  the  desired  shade  of  colour,  it  is  dropped 
into  a  tub  of  water  and  then  crushed  to  powder.  The 
calcination  requires  great  experience  on  the  part  of 
the  operator,  because  so  long  as  the  product  is  hot,  it 
has  quite  a  different  colour  from  that  assumed  on 
complete  cooling.  Since  only  comparatively  small 
quantities  of  ochre  can  be  treated  in  this  way,  and  the 
operation  unnecessarily  increases  the  cost  of  the 
product,  owing  to  the  large  consumption  of  fuel,  it  is 
highly  desirable  to  employ  a  simple  calcining  apparatus 
capable  of  treating  large  quantities. 

Such  an  apparatus  may  consist  of  an  iron  drum, 
mounted  with  a  gentle  slope  inside  a  furnace,  from 
which  it  projects  at  both  ends.  A  shaft  carrying  a 
sheet  metal  worm  is  rotated  inside  the  drum;  and  the 
whole  apparatus  is  very  similar  to  an  Archimedean 

When  the  iron  drum  is  raised  to  a  strong  red  heat, 
and  small  quantities  of  ochre  are  fed  continuously  into 
the  upper  end  of  the  drum,  the  rotation  of  the  worm 
will  push  the  material  forward,  and  contact  with  the 
glowing  sides  of  the  drum  will  produce  the  necessary 
calcination,  the  degree  of  which  can  be  modified  by 
altering  the  speed  at  which  the  worm  is  turned.  The 
calcined  product  is  discharged  at  the  lower  end  of  the 
drum,  either  into  a  vessel  of  water,  or,  if  only  moderate 
heating  has  been  applied,  direct  into  a  collector. 

Fig.  28  represents  an  apparatus  designed  by  Halliday 
for  the  dry  distillation  of  wood  waste ;  but,  with  slight 
structural  modifications,  it  can  also  be  used  for  calcining 
ochre.  The  material  to  be  heated  is  introduced,  in 
small  pieces,  into  the  feed  hopper  B,  and  is  carried 
downward,  by  the  worm  C,  into  the  red-hot  drum  A, 
through  which  it  is  propelled  by  the  worm  D  until  it 



drops  out,  at  F,  into  the  tank  G.  The  length  of  time 
the  material  is  subjected  to  calcination  depends  on  the 
speed  at  which  the  worm  D  is  run.  The  pipe  E  carries 
off  the  water  vapour  expelled  from  the  charge. 

In  order  to  obtain  a  uniform  product  when  ochre  is 
calcined  in  an  apparatus  constructed  on  this  principle, 
it  is  necessary  that  the  material  introduced  should  be 

FIG.  28. 

fairly  regular  in  size,  a  condition  which  is  easily  fulfilled 
by  squeezing  the  freshly  dug  ochre  between  fluted 
rollers,  and  then  passing  it  over  a  series  of  screens,  each 
grade  being  then  calcined  separately. 

Moreover,  the  apparatus  is  only  suitable  for  calcining 
at  medium  temperatures;  and  when  highly  calcined 
products  are  in  question,  the  operation  is  best  performed 
in  fire-clay  cylinders,  or  in  thick  cast-iron  drums,  similar 
to  gas  retorts,  built  into  a  furnace. 


Other  devices  for  calcining  ochre  will  be  described 


As  previously  stated,  ochres  are  frequently  met  with 
in! Nature,  both  in  the  immediate  vicinity  of  iron  ore, 
and  also  at  considerable  distances  from  such  deposits. 
In  the  latter  case,  the  ochre  must  be  assumed  to  be 
the  decomposition  products  of  ferruginous  minerals 
and  to  have  been  carried  off  by  water  until  the  latter 
became  stagnant  and  allowed  the  ochre  to  settle  down. 
In  their  method  of  deposition  these  ochres  are  therefore 
analogous  to  clay,  and  they,  too,  often  contain  large 
quantities  of  extraneous  minerals,  which  have  given 
rise  to  the  diversified  substances  grouped  under  the 
name  of  ochre. 

Although  ochres  are  so  widespread  in  Nature,  only 
certain  kinds,  found  in  certain  localities,  have  acquired 
a  high  reputation.  For  the  most  part,  these  ochres 
are  such  as  have  already  been  prepared  in  a  high 
degree,  by  Nature,  for  the  purpose  for  which  they  are 

Thus,  we  find  that  all  the  ochres  which  have  acquired 
a  high  repute  among  painters  for  particular  beauty 
of  tone  and  permanence,  are  distinguished  by  two 
properties  :  a  high  content  of  ferric  hydroxide  and  great 

The  former  of  these  properties  imparts  brightness 
of  colour;  and  such  products  will  furnish,  on  calcina- 
tion, a  wide  range  of  colour  shades.  When,  as  is  the 
case  with  the  finer  qualities  of  ochre,  the  mineral 
contains  only  a  very  small  proportion  of  impurities, 
there  is  no  difficulty  in  bringing  it,  by  simple  grinding 


or  levigation,  into  a  condition  in  which  it  is  at  once 
fit  for  use  as  a  pigment. 

The  Italian  ochres  have,  for  long  ages,  enjoyed  a 
high  reputation  for  their  beauty  of  colour  and  per- 
manence. This  category  includes,  for  example,  the 
renowned  Siena  earth,  Roman  earth,  Italian  umber, 
and  other  ochre  colours.  This  high  renown  is  probably 
due  less  to  the  inherent  properties  of  the  mineral  than 
to  the  circumstance  that  the  art  of  painting  attained 
a  high  state  of  development  at  an  early  period,  and 
that  the  artists  paid  special  attention  to  the  use  of 
bright  and  permanent  colours  for  their  work. 

Although,  at  present,  many  deposits  of  ochre  are 
known  that  are  quite  able  to  compete,  on  the  score  of 
beauty,  with  the  best  Italian  products,  the  good  name 
of  these  latter  has  nevertheless  been  maintained.  It 
is  true  that  the  name  of  Italian  ochre  is  often  merely 
borrowed,  for  application  to  a  product  originating  in 
some  other  country,  varieties  of  terra  di  Siena,  for 
instance,  being  put  on  the  market  that  have  actually 
been  derived  from  deposits  in  Germany. 

As  a  result  of  this  custom,  certain  names,  such  as 
terra  di  Siena,  umbra  di  Roma,  have  become  generic 
terms,  and  their  use  denotes,  not  an  intention  to  suggest 
that  the  earth  colours  in  question  really  come  from 
Siena  or  the  vicinity  of  Rome,  but  that  the  properties 
of  the  article  are  equal  to  those  of  the  old-established 
colours  of  Siena  or  Rome. 

It  would  occupy  too  much  space  to  go  into  an 
exhaustive  description  of  all  the  native  varieties  of 
ochre,  and  would  inevitably  lead  to  a  good  deal  of 
repetition.  It  will  therefore  be  sufficient,  for  our 
purpose,  to  deal  with  only  a  few  of  them. 

The  best -known  ochres  are  those  of  Rome  and  Siena, 


the  latter  being  frequently  called,  in  commerce,  by  its 
Italian  name,  terra  di  Siena. 

Roman  ochre  forms  yellowish-brown  masses,  of 
fairly  fine  texture  and  composed  of  ferric  hydroxide 
and  clay.  They  are  put  on  the  market  both  in  the 
raw  and  calcined  state.  On  calcination,  the  colour 
soon  changes  to  red,  and  if  carefully  performed,  the 
resulting  colours  have  a  very  warm,  fiery  tone. 

Closely  approaching  Roman  earth  is  the  English 
ochre,  which  is  worked  more  particularly  in  Surrey, 
and  is  not  infrequently  sold  as  Roman.  In  many 
deposits  this  English  ochre  occurs  in  such  a  high  state 
of  purity  that  the  best  pieces  are  picked  out  and  sold 
without  being  even  crushed  or  ground.  The  pieces 
of  lower  quality  are  very  carefully  ground  and  levigated, 
for  the  purpose  of  being  calcined  for  the  production  of 
different  shades,  and  then  furnish  highly  prized  colours. 

In  point  of  chemical  composition,  the  ochre  family 
also  includes  terra  di  Siena,  bole,  umber  and  Cassel 
brown.  These  minerals,  however,  are  not  yellow  like 
ochre,  but  brown,  and  will  therefore  be  dealt  with 
along  with  the  brown  earth  colours. 


Products  very  similar,  both  in  chemical  composition 
and  colour,  to  the  native  ochres  can  also  be  very  simply 
and  cheaply  made  by  artificial  means.  Their  prepara- 
tion may  be  particularly  recommended  to  colour- 
makers  who  desire  to  turn  out  a  wider  range  of  iron 
pigments,  but  are  not  in  a  position  to  obtain  natural 
ochres  at  a  low  price. 

In  the  manufacture  of  artificial  ochre,  an  endeavour 
is  made  to  imitate  the  natural  processes  which  have 


led  to  the  formation  of  ochre,  and,  of  course,  to  avoid 
anything  likely  to  hinder  the  production  of  a  suitable 
colour  earth,  for  example  the  presence  of  sand  or  a 
considerable  admixture  of  extraneous  minerals. 

As  already  mentioned,  the  chief  impurities  in  natural 
ochres  are  clay  and  sand,  both  of  which  can  be  easily 
excluded  during  the  manufacture  of  artificial  ochre, 
or  their  amount  controlled  in  such  a  manner  that  paler 
or  darker  products  can  be  obtained  at  will,  and  the 
tone  varied,  in  any  desired  manner,  by  calcination,  as 
in  the  case  of  the  native  article. 

The  raw  material  for  artificial  ochre  is  always  a 
ferrous  salt,  which  can  be  purchased  in  large  quantities 
and  at  very  low  prices,  namely  green  vitriol,  which, 
in  the  pure  state,  consists  of  ferrous  sulphate,  FeSO4 
-f  7H2O.  This  substance  forms  sea-green  crystals, 
which  are  readily  soluble  in  water  and  impart  an 
objectionable  inky  flavour  thereto.  On  exposure  to 
the  air,  green  vitriol  turns  an  ugly  brown  colour,  and 
is  no  longer  completely  soluble  in  water,  passing  gradu- 
ally into  the  condition  of  basic  ferrous  sulphate.  This 
is  because  ferrous  oxide  is  a  highly  unstable  substance, 
which  attracts  oxidation  and  changes  into  ferric  oxide. 
This  latter,  however,  requires  for  the  production  of 
soluble  salts  a  larger  quantity  of  acids  than  does  ferrous 
oxide,  and  therefore  the  oxidation  of  ferrous  sulphate 
in  the  air  leads  only  to  the  formation  of  salts  that  are 
imperfectly  saturated  with  acid,  namely  basic  salts. 

When  a  solution  of  green  vitriol  is  left  exposed  to 
the  air,  basic  ferric  sulphate  is  also  formed,  which 
settles  down  to  the  bottom  of  the  vessel  as  a  rusty 
powder.  If,  however,  a  corresponding  quantity  of 
sulphuric  acid  be  added  to  the  solution  at  the  outset, 
the  resulting  ferric  sulphate  remains  in  solution. 


On  treating  the  green  vitriol  solution  with  one  of 
caustic  potash,  caustic  soda  or  quick  lime,  the  ferrous 
oxide  is  thrown  down  as  the  corresponding  hydroxide, 
forming  a  voluminous  greyish-green  precipitate.  This 
hydroxide  still  possesses  a  great  affinity  for  oxygen, 
and  when  the  precipitate  is  brought  into  contact  with 
air,  its  colour  rapidly  changes  to  a  rusty  red,  through 
the  transformation  of  the  ferrous  hydroxide  into  ferric 
oxide.  The  ferrous  hydroxide  can  also  be  precipitated 
by  alkali  carbonates,  the  deposits  behaving  in  exactly 
the  same  manner  as  that  thrown  down  by  the  caustic 

Various  methods  can  be  adopted  in  the  preparation 
of  artificial  ochre,  the  selection  depending  on  the 
properties  desired  in  the  finished  product.  To  obtain 
an  ochre  with  particularly  good  covering  power,  the 
method  must  be  different  from  that  employed  to  furnish 
a  cheap  product,  in  which  low  price  is  more  important 
than  covering  power. 

In  the  former  case,  the  ferrous  hydroxide  is  mixed 
with  substances  which,  in  themselves,  possess  fairly 
high  covering  power,  such  as  chalk  or  white  clay;  in 
the  second,  gypsum,  which  is  of  low  covering  power, 
is  used. 

The  preparation  of  the  cheapest  kinds  of  artificial 
ochre  will  be  described  first,  followed  by  that  of  the 
higher  grades  which  belong  to  the  most  valued  artists' 

For  cheap  artificial  ochres,  the  ferrous  hydroxide 
is  thrown  down  by  caustic  lime  from  a  solution  of 
green  vitriol.  According  as  a  lighter  or  darker  shade  is 
required,  two  to  three  parts  of  ferrous  sulphate  are 
dissolved  in  water,  care  being  taken  to  select  crystals 
of  a  pure  green  colour,  since  those  that  have  a  rusty 


look  are  only  imperfectly  soluble,  because  they  contain 
basic  ferric  sulphate. 

The  solution  will  always  be  cloudy,  owing  to  the 
partial  precipitation  of  the  hydroxide  by  the  lime  in 
the  water;  but  this  is  immaterial.  For  the  precipita- 
tion, a  milk  of  lime  is  prepared  by  slaking  one  to  two 
parts  of  quicklime  (according  to  the  quantity  of  ferrous 
sulphate  to  be  treated)  in  water,  and  stirring  this  up 
in  enough  water  to  make  a  thin  milk.  Care  must  be 
taken  to  exclude  any  large  particles  of  lime,  since  these 
would  find  their  way  into  the  finished  product  and 
make  the  colour  uneven.  On  this  account,  the  milk 
of  lime  should  be  carefully  strained  through  a  loosely 
woven  cloth  or  fine  sieve,  into  the  precipitation 

The  ferrous  sulphate  solution  is  then  poured  in,  the 
mixture  being  kept  stirred,  and  an  ugly,  grey-green 
precipitate  is  produced,  consisting  of  a  mixture  of 
ferrous  hydroxide  and  calcium  sulphate,  the  reaction 
being  explained  by  the  equation  :— 

FeSO4  +  Ca(OH)2  -  Fe(OH)2  +  CaSO4. 

The  larger  the  amount  of  ferrous  sulphate  solution 
added  to  the  milk  of  lime,  the  darker  the  resulting 
ochre.  As  soon  as  all  the  ferrous  sulphate  is  in,  the 
stirring  is  suspended,  and  the  liquid  is  left  until  quite 
clear.  The  water  is  drawn  oif  through  tapholes  in 
the  side  of  the  vessel,  care  being  taken  not  to  disturb 
the  fine  precipitate,  and  fresh  water  is  added,  in  which 
the  deposit  is  stirred  up  and  again  left  to  settle  down. 
This  operation,  which  is  once  or  twice  repeated,  is  to 
wash  the  precipitate. 

When  this  object  has  been  sufficiently  accomplished, 
the  mass  is  shovelled  out  of  the  vessel  and  spread 


thinly  on  boards,  where  it  is  left  until  the  desired  shade 
of  colour  has  been  attained,  the  colour  changing  quickly 
on  exposure  to  air,  owing  to  the  oxidation  of  the  ferrous 
hydroxide  into  ferric  hydroxide.  To  ascertain  whether 
oxidation  is  complete,  a  large  lump  of  the  mass  is 
broken  across ;  and  if  it  is  of  a  uniform  yellow-brown 
colour  throughout,  without  being  darker  on  the  outside 
than  in  the  middle,  all  the  ferrous  hydroxide  will  have 
been  transformed  into  the  ferric  state.  The  product 
can  now  be  dried  at  once,  and  when  ground  will  be 
ready  for  sale. 

To  obtain  different  varieties  from  the  product,  it 
is  carefully  heated  (in  a  finely  powdered  condition)  in 
shallow  pans;  but  the  operation  needs  caution,  or  the 
water  in  the  gypsum  present  will  be  expelled,  giving 
rise  to  drawbacks  that  are  manifested  when  the  colour 
is  used. 

For  instance,  in  mixing  such  a  colour  with  water, 
the  gypsum  would  again  absorb  water  and  cause  the 
whole  mass  to  set  as  a  useless  solid  lump.  Since 
gypsum  parts  with  its  water  at  a  comparatively  low 
temperature,  it  is  better  not  to  heat  these  cheap  ochres 
at  all,  but  to  obtain  the  various  shades  by  modifying 
the  proportion  of  ferrous  sulphate  employed. 

Another  defect  of  the  ochres  prepared  by  this  method 
resides  in  the  excess  of  lime  present,  it  being  impracti- 
cable to  measure  out  the  quantity  of  lime  used  with 
such  accuracy  that  only  just  enough  is  taken  to  pre- 
cipitate the  ferrous  hydroxide,  there  being  always  a 
slight  excess.  This  lime  is  transformed  into  calcium 
carbonate  on  the  mass  being  exposed  to  the  air,  just 
as  in  the  preparation  of  Vienna  white;  but  as  the 
saturation  with  carbon  dioxide  takes  a  considerable 
time,  some  of  the  lime  remains  in  the  caustic  state 


and  is  liable  to  affect  other  colours  that  may  be  mixed 
with  the  ochre. 

An  artificial  ochre  uniting  in  itself  all  the  qualities 
of  the  natural  product,  and  also  capable  of  being  shaded 
by  burning,  can  be  prepared  in  the  following  manner. 
An  accurately  weighed  quantity  of  pure  crystallised 
ferrous  sulphate  is  dissolved  in  a  definite  amount  of 
water,  and  the  solution  is  treated  with  successive  small 
portions  of  crude  nitric  acid,  until  all  the  ferrous  oxide 
has  been  changed  into  the  ferric  state.  The  change 
can  be  detected  by  a  very  decisive  test.  If  a  liquid 
containing  ferric  oxide  in  solution  is  brought  into 
contact  with  a  solution  of  red  prussiate  of  potash 
(potassium  f erri cyanide) ,  no  precipitate  is  formed  in 
the  absence  of  ferrous  oxide,  but  only  a  brown  colora- 
tion; whereas,  if  ferrous  oxide  is  present,  a  beautiful 
blue  precipitate  is  formed  at  once,  the  colour  of  which 
is  so  intense  that  very  small  quantities  of  ferrous  oxide 
can  be  detected  by  this  means. 

For  the  purpose  now  under  consideration,  the 
presence  of  small  amounts  of  ferrous  oxide  in  the 
solution  is  immaterial,  because  they  are  soon  changed 
into  ferric  oxide  on  exposure  to  the  air.  It  might, 
therefore,  be  asked,  why  take  the  trouble  to  oxidise 
the  ferrous  oxide  by  means  of  an  agent  involving 
expense,  which  could  be  saved  by  allowing  the  oxidation 
to  take  place  in  the  air? 

The  advantage,  however,  of  the  direct  employment 
of  a  solution  of  ferric  oxide  is  that  it  gives  at  once  a 
colour  that  can  be  dried  straight  away;  wrhilst  at  the 
same  time  the  colour  undergoes  no  change  in  drying, 
whereas  it  does  when  ferrous  oxide  solution  is  used. 

The  method  of  producing  ochres  from  this  ferric 
solution  varies  according  as  the  product  is  to  be  used 


without  any  further  treatment  than  drying,  or  is  to  be 
modified  by  firing. 

In  the  former  event,  caustic  lime  is  again  used  as 
the  precipitant,  but  in  only  just  sufficient  quantity  to 
throw  down  all  the  ferric  oxide  in  the  solution.  This 
amount  can  be  calculated  exactly,  36-84  parts  by  weight 
of  pure  burnt  lime  being  required  for  every  100  parts 
of  pure  ferrous  sulphate  taken.  The  actual  quantity, 
whether  larger  or  smaller,  will  depend  on  the  relative 
purity  of  the  sulphate  and  lime ;  and  this  can  readily 
be  ascertained  by  a  simple  trial. 

The  lime  is  used  in  the  form  of  milk  of  lime,  as 
already  described.  If  lime  alone  is  employed,  the 
precipitate  will  consist  of  pure  ferric  hydroxide  and  the 
calcium  sulphate  thrown  down  at  the  same  time.  The 
resulting  colour,  when  dried,  will  be  an  intensely  brown 
mass,  which  can  be  used  in  place  of  the  very  dark 
natural  ochres. 

In  order  to  obviate  entirely  the  disadvantages 
resulting  from  the  presence  of  a  large  amount  of  caustic 
lime  in  the  precipitate,  fine  levigated  chalk  or  white 
clay  is  added  in  the  preparation  of  the  lighter  shades 
of  ochre,  the  addition  being  made  as  soon  as  the  two 
ingredients  have  been  brought  into  contact;  and  the 
mixture  is  thoroughly  stirred,  to  ensure  uniform 
admixture  with  the  ferric  hydroxide.  The  colour  of 
the  settled  deposit  will  be  lighter  or  darker  in  propor- 
tion to  the  amount  of  chalk  or  clay  employed;  and  in 
this  way  the  whole  range  of  shades  from  pale  yellow 
to  bright  brown  can  be  obtained  without  the  application 
of  heat. 

Ochre  that  has  been  made  with  chalk  is  unsuitable 
for  toning  by  heat,  because  this  treatment  would 
causticise  the  lime,  and  the  ochre  could  not  be  mixed 


with  other  colours,  since  these  would  be  affected  by  that 
substance.  On  the  other  hand,  when  white  clay  is 
used  in  preparing  the  ochre,  the  latter  can  be  more 
easily  toned  by  firing,  provided  care  be  exercised  in  the 
process.  The  ochre  must  be  dried  completely  in  the 
air,  and  either  spread  out  in  thin  layers  on  iron  plates, 
for  the  burning  process,  or  else  put  into  a  drum,  of  the 
kind  already  described,  in  which  the  mass  is  moved 
onward  by  a  worm. 

The  clay  remains  unaltered  in  firing,  but  the  gypsum 
parts  with  its  water  of  crystallisation.  In  order  to 
restore  the  latter,  the  ochre  issuing  from  the  drum  is 
discharged  direct  into  a  vessel  of  water,  in  which  it 
can  be  kept  in  constant  motion  by  a  stirrer.  The 
water  is  soon  warmed  by  the  heat  of  the  mass,  and 
absorption  by  the  gypsum  proceeds  at  a  rapid  rate. 
When  the  whole  charge  has  been  fired  and  collected  in 
the  vessel  of  water,  the  stirrer  is  stopped  and  the 
precipitate  dried,  being  then  ready  for  use. 

In  certain  circumstances,  ochre  can  be  made  by  other 
methods.  In  large  towns,  ammonium  salts  are  some- 
times obtainable  at  a  moderate  price,  being  manu- 
factured in  large  quantities  as  a  by-product  in  gasworks. 
For  our  purpose,  crude  gas  liquor  might  be  used,  since 
it  contains  ammonia  for  the  precipitation  of  the 
ferric  hydroxide.  In  most  cases,  however,  this  gas 
liquor  contains  only  very  small  quantities  of  ammonia, 
and,  therefore,  in  a  works  of  any  size,  very  large  vessels 
would  be  needed  for  the  production  of  a  comparatively 
small  quantity  of  ochre.  On  this  account,  preference 
is  given  to  crude  carbonate  of  ammonia,  which  is  also 
obtainable  at  low  prices. 

On  bringing  a  solution  of  this  salt  into  contact  with 
one  of  ferric  oxide,  ferric  hydroxide  is  precipitated, 


and  the  sulphate  of  ammonia  resulting  from  the 
reaction  remains  in  solution.  By  stirring  white  clay 
into  the  liquid  at  the  same  time,  the  ochre  can  be 
correspondingly  lightened  in  shade. 

The  precipitates  obtained  in  this  way  can  be  dried 
at  once,  and  converted  into  any  shade  obtainable 
with  natural  ochre,  from  brown  to  red,  by  strong 
firing.  The  sulphate  of  ammonia  still  remaining  in  the 
air-dried  product  is  completely  volatilised  by  the  heat, 
and  the  resulting  ochres  are  even  superior  to  the 
natural  varieties  in  beauty  and  permanence. 


In  the  manufacture  of  certain  chemicals,  substances 
of  divergent  composition  are  obtained  which  are  sold 
under  the  name  of  ochre  and  are  used  as  painters' 
colours.  Whereas  ochre,  properly  so-called,  consists 
of  either  ferric  hydroxide  or  ferric  oxide  in  association 
with  clay,  lime,  etc.,  the  products  now  under  considera- 
tion are  basic  ferric  salts  composed  of  varying  quantities 
of  ferric  oxide  in  combination  with  certain  proportions 
of  sulphuric  acid. 

These  ochres  are  obtained  as  by-products  in  the 
manufacture  of  green  vitriol  from  pyrites,  and  in  alum 
manufacture ;  and,  according  to  their  origin,  they  are 
classed  as  vitriol  ochre,  so-called  alum  sludge,  and  pit 
ochre.  All  the  basic  ferric  sulphates  of  which  they  are 
composed  form  fairly  large  crystals,  and,  therefore,  in 
most  cases,  the  covering  power  is  small.  On  this 
account  the  products  are  of  low  grade  and  are  put 
on  the  market  at  low  prices,  for  which  reason  they  are 
largely  used  in  making  cheap  paints. 

Vitriol  Ochre. — Commercial  green  vitriol  is,  for  the 


most  part,  manufactured  from  native  sulphides  of 
iron.  When  many  of  these  sulphides  are  piled  in 
heaps  and  left  to  the  action  of  the  air,  oxygen  is  gradu- 
ally absorbed  and  green  vitriol  is  formed  which  is 
dissolved  out  by  rain  and  is  collected  in  large  clarifying 

In  the  case  of  pyrites,  however,  the  mineral  must 
first  be  roasted  in  a  current  of  air,  since  otherwise  its 
conversion  into  green  vitriol  would  only  proceed  in  a 
very  sluggish  manner.  In  any  event,  the  aqueous 
solution  of  ferrous  sulphate  has  to  be  concentrated, 
by  evaporation,  to  the  point  at  which  the  green  vitriol 
crystallises  out. 

Both  in  the  clarifying-tanks  and — still  more  so — in 
the  evaporating-pans,  a  rusty-looking  sediment  forms 
at  the  bottom,  consisting  of  basic  ferric  sulphate. 
This  originates  in  the  partial  oxidation  of  the  ferrous 
oxide  (first  formed)  while  the  pyrites  is  exposed  to  the 
air,  and  since  the  quantity  of  sulphuric  acid  present  is 
insufficient  to  saturate  all  the  ferric  oxide,  basic  salts 
are  produced. 

The  yellow-brown  sludge  deposited  in  the  pans 
during  the  concentration  of  crude  green  vitriol  liquor, 
constitutes  the  product  termed  vitriol  ochre,  which 
contains  varying  amounts  of  ferric  oxide,  sulphuric 
acid  and  water,  according  to  the  quantity  of  ferric 
oxide  resulting  from  the  oxidation  of  the  pyrites  and 
the  character  of  the  latter,  e.  g,  :— 

*  Ferric  oxide  ....  65-70% 
Sulphuric  acid  ....  14-16% 
Water 13-16% 

Although  the  colour  of  these  ochres  is  not  particularly 
handsome,  they  can  be  transformed,  by  firing,  into 
colours  of  fairly  good  quality.  As  this  subject  will  be 


more  thoroughly  gone  into  when  dealing  with  the 
preparation  of  the  red  iron  pigments,  the  applicability 
of  these  ochres  will  only  be  casually  referred  to  here. 
During  the  burning  process,  these  ochres,  of  course, 
part  with  the  whole  of  their  contained  water;  and  by 
protracted,  high  calcination,  the  whole  of  the  sulphuric 
acid  can  also  be  expelled,  so  that  finally  nothing  but 
pure  ferric  oxide  is  left. 

Alum  Sludge. — Solutions  of  crude  alum  always 
contain  a  certain  amount  of  ferric  oxide  which  settles 
down  at  the  bottom  of  the  pans  during  concentration. 
This  sludge,  too,  consists  of  basic  ferric  sulphate,  but 
is  inferior  in  covering  power  to  vitriol  ochre,  the 
crystals  being  of  coarser  grain.  On  the  other  hand, 
the  ochreous  sediment  from  the  alum  concentrating- 
pans  has  the  valuable  property  of  being  readily 
transformable  into  red-brown  to  pure  red  tones  by 
burning.  For  this  reason,  particular  attention  has 
been  devoted  to  this  sludge  in  a  number  of  alum  works. 

Since  the  products  are  only  of  value  when  burned, 
and  the  shades  thereby  obtained  are  always  red,  they 
will  be  dealt  with  more  fully  along  with  the  red  earth 

Pit  Ochre. — Springs  containing  small  quantities  of 
ferrous  sulphate  and  other  salts  are  met  with  in  many 
iron  mines,  but,  in  most  cases,  the  amounts  are  too 
small  for  their  recovery  by  artificial  concentration  to 
be  contemplated.  If,  however,  the  conditions  allow 
of  the  springs  being  easily  diverted,  they  may  often  be 
utilised  for  the  preparation  of  low-grade  ochre. 

The  chemical  composition  of  these  pit  ochres  varies 
considerably,  and  depends  on  the  geological  character 
of  the  locality.  Water  can  only  dissolve  such  minerals 
as  occur  in  the  form  of  fairly  readily  soluble  com- 


pounds;  and  for  this  reason  pit  waters  are  always 
solutions  of  the  metals  which  are  found  in  the  mine. 

The  variety  of  compounds  that  may  be  present  in  an 
ochre  can  be  seen  from  the  subjoined  analyses  of 
ochres  deposited  from  pit  waters  at  Rammelsberg. 
As  elsewhere,  two  distinct  classes  of  ochre  are  met  with, 
having  a  conchoid  and  an  earthy  fracture  respectively. 
The  latter  usually  contain  rather  more  ferric  oxide, 
and,  in  particular,  a  higher  content  of  foreign  sub- 
stances, the  most  important  of  which  is  quartz  sand. 
In  the  Table,  the  ochres  with  conchoid  fracture  are 
marked  A,  and  those  with  an  earthy  fracture,  B. 

A.  B. 

Ferric  oxide  .          .          .      68-75  63-85 

Zinc  oxide  .  ...        1-29  1-23 

Copper  oxide  .          .          .       0-50  0-88 

Sulphuric  acid  .          .          .9-80  J3'59 

Water          .  ...      15*52  18*45 

Clay  and  Quartz  .          .          .       4-14  2-00 

The  preparation  of  the  ochre  is  a  simple  matter, 
consisting  in  collecting  the  mass  and  sorting  out  the 
loose,  earthy  portions  of  a  pure  yellow  colour  from  the 
denser  and  darker  parts.  The  former  are  dealt  with 
separately,  usually  by  a  simple  process  of  levigation, 
for  the  sole  purpose  of  getting  rid  of  the  earthy  matter, 
quartz  sand  in  particular. 

The  denser  varieties  require  much  more  work,  but 
yield  a  far  superior  product,  which,  by  suitable  treat- 
ment, can  be  converted  into  the  finest  grades  of  ochre. 
The  first  operation  consists  in  a  very  careful  crushing, 
and  as  the  pieces  are  often  very  hard,  they  are  treated 
in  ordinary  or  stamp-mills,  edge-runners  being  also 
employed  with  advantage. 

The  product  reduced  by  any  of  these  means  is  passed 
through  a  number  of  sieves,  to  separate  the  fine 


particles  from  the  coarse;  and  the  finest  dust  is  burnt. 
This  last  treatment  causes  a  considerable  loss  in  weight, 
both  the  accompanying  water  and  most  of  the  sulphuric 
acid  being  volatilised.  However,  since,  as  already 
stated,  all  varieties  of  ochre  can  be  obtained,  the 
process  is  consequently  very  remunerative  notwith- 
standing the  loss  in  weight  it  involves. 

Yellow  Earth. — From  the  particulars  given  in  the 
general  description  of  the  earth  colours,  yellow  earth 
may  also  be  regarded,  to  some  extent,  as  an  ochre, 
but  one  containing  a  large  proportion  of  foreign  sub- 
stances. It  might,  however,  be  more  accurately 
termed  a  clay  contaminated  with  a  considerable  amount 
of  quartz  sand  and  a  certain  proportion  of  ferric  oxide. 
The  method  of  preparation  is  on  the  same  lines  as  for 
ochre,  but  burning  is  never  practised,  nor  is  the  treat- 
ment so  careful  as  for  the  better  grades  of  ochre,  the 
low  price  of  the  colour  making  this  unremunerative. 



THE  number  of  minerals  that  can  be  directly  used 
as  red  earth  pigments  is  comparatively  small,  and  by 
far  the  greater  proportion  consist  of  ferruginous  colours, 
a  few  of  which  are  obtained  by  the  mechanical  treat- 
ment of  native  iron  ores  or  clays  coloured  red  by  ferric 
oxide,  the  majority,  however,  being  formed  by  burning 
certain  materials  of  another  colour.  To  these  belong 
nearly  all  the  materials  mentioned  in  connection  with 
the  ochres  and  the  brown  iron  colours,  together  with 
a  few  by-products  of  the  chemical  industry. 

In  addition  to  the  foregoing,  which  have  ferric  oxide 
for  their  pigmentary  principle,  is  the  native  mercury 
sulphide,  occurring,  as  scarlet,  crystalline  masses,  under 
the  name  of  cinnabar  (vermilion).  The  only  reason 
for  including  natural  vermilion  with  the  earth  colours 
is  to  make  the  list  complete,  the  largest  proportion 
of  this  pigment  being  prepared  by  artificial  methods. 
The  product  sold  as  "  Chinese  "  vermilion  may,  in 
former  times,  have  really  been  introduced  from  China 
into  Europe,  and  prepared  there  by  grinding  and 
levigating  the  best-coloured  lumps  of  the  natural 
cinnabar;  but,  at  the  present  time,  all  the  vermilion 
made — in  Euro  peat  least — is  from  sulphur  and  mercury, 
by  artificial  processes,  and  the  name  Chinese  vermilion 
is  merely  retained  to  designate  a  particularly  fine  grade. 


On  the  basis  of  occurrence  and  chemical  properties, 
the  red  earths  can  be  classified  into  several  groups. 
The  first  comprises  natural  products  requiring  only 
mechanical  preparation,  such  as  the  minerals  known  as 
hematite,  micaceous  iron  ore,  Elbaite,  etc.,  and  the 
special  modification  of  red  ironstone  termed  raddle. 
All  these  minerals  consist  almost  entirely  of  ferric 
oxide  in  a  pure  state.  The  mineral,  bole  (red  chalk, 
terra  sigillata,  Lemnos  earth),  is  chemically  allied  to  the 
ochres,  being,  like  them,  composed  of  alumina,  fre- 
quently accompanied  by  lime  and  small  quantities  of 
magnesia,  but  differing  in  that  ferric  oxide  is  always 
present  in  bole,  whereas  the  ochres  always  contain 
ferric  hydroxide. 

The  second  group  consists  of  the  artificial  reds 
obtained  by  burning  or  calcining  raw  materials,  whose 
ferric  hydroxide  is  more  or  less  transformed  by  heat 
into  ferric  oxide,  such  as(  vitriol  ochre,  pit  ochre  and 
alum  sludge. 

Of  late  years  the  artificial  earth  colours  have  attained 
a  high  degree  of  importance.  They  are  obtained  in 
large  quantities  in  the  manufacture  of  sulphuric  acid 
from  green  vitriol.  Formerly,  it  is  true,  they  were 
also  used  as  pigments  under  the  name  of  caput  mortuum 
or  colcothar,  but  were  not  held  in  much  esteem;  and 
it  is  only  within  recent  times  that  it  has  been  discovered 
that  these  inferior  by-products  can  be  converted  into 
very  handsome  and  brilliant  colours,  which  now  form 
important  articles  of  commerce. 


Bole,  Lemnos  earth,  terra  sigillata,  etc.,  is,  for  the 
most  part,  a  product  of  the  decomposition  of  highly 


ferruginous  minerals,  and  occurs,  in  the  form  of  lumps, 
having  a  conchoidal  fracture,  in  pockets  or  detritus. 
The  lumps  have  a  sp.  gr.  of  2-2-2'5,  are  Isabella  brown 
to  dark  brown  in  colour,  and  give  a  slightly  greasy- 
looking  streak.  There  are  two  distinct  varieties  of 
bole  :  the  one  adhering  firmly  to  the  tongue,  whilst 
the  other  lacks  this  property  and,  when  placed  in  water, 
crumbles  down  to  powder  in  emitting  a  peculiar  noise. 

The  composition  of  the  boles  varies,  but  all  of  them 
may  be  regarded  as  alumino  ferric  silicates  combined 
with  water.  Most  of  the  specimens  examined  from 
different  deposits  contain  24-25%  of  water,  41-42% 
of  silica,  and  20-25%  of  alumina,  the  remainder  con- 
sisting of  ferric  oxide  with  small  traces  of  manganese 

Some  varieties,  however,  are  exceptional  and  contain 
only  30-31%  of  silica  and  17-21%  of  water,  e.  g.  those 
from  Orawitza  and  Sinope.  Lemnos  earth,  the  true 
terra  sigillata,  is  mostly  silica  (66%)  with  8%  of  water, 
and  contains  a  smaller  percentage  of  ferric  oxide  than 
the  others.  It  is  also  of  a  distinct  colour,  lighter  than 
the  true  boles  and  having  a  greyish  or  yellowish  tinge. 

The  behaviour  of  the  different  kinds  on  burning  is 
just  as  diverse  as  their  chemical  composition.  Whilst 
some  kinds  are  infusible  at  even  the  highest  tempera- 
tures, and  merely  change  into  hard,  red  masses;  others, 
again,  fuse  at  a  moderate  heat.  This  difference  is  due 
to  chemical  composition,  those  high  in  silica  being 
generally  less  refractory  than  those  in  which  alumina 

In  order  to  render  the  boles  suitable  for  painting, 
they  are  put  through  a  somewhat  different  treatment 
than  the  other  earth  colours.  The  freshly  dug  material 
is  first  sorted,  the  uniformly  coloured  lumps  of  fine 


texture  being  set  apart  and  suffused  with  water,  with 
which  they  form  a  pasty  mass  of  low  plasticity,  which 
is  kneaded  by  hand  to  make  it  homogeneous,  and  is 
then  stirred  up  with  more  water.  When  the  lumps 
have  distributed  in  the  water,  the  latter  is  drawn  off 
into  a  second  tub,  and  the  residue  is  stirred  up  with 
fresh  water,  the  treatment  being  repeated  until  the 
effluent  no  longer  shows  any  signs  of  colour. 

The  liquid  in  which  the  finely  divided  bole  is  sus- 
pended is  left  to  settle,  and  the  bole  subsides  as  a  fine 
powder,  which  is  dried  to  the  condition  of  paste, 
pressed  into  moulds  and  dried  completely. 

Owing  to  its  low  content  of  ferric  oxide,  the  colour 
of  bole  is  not  particularly  bright,  but  is  very  permanent 
— a  property  equally  shared  by  all  the  other  ferric 
oxide  pigments. 


In  nature,  ferric  oxide  forms  extensive  deposits, 
which,  by  reason  of  the  light  red  colour  characteristic 
of  certain  varieties  of  ferric  oxide,  are  largely  employed 
in  painting.  These  colours  may  be  classed  among  the 
oldest  known  to  mankind,  ferric  oxide  pigments  having 
been  used  frequently  in  the  most  ancient  paintings. 

The  most  important  varieties  of  ferric  oxide  for  our 
purpose  are :  iron  glance,  with  its  modifications, 
micaceous  iron  ore  and  frothy  hematite ;  red  hematite, 
and  raddle. 


This  substance  forms  handsome  black  crystals  of 
very  high  lustre,  which,  when  small  and  scaly,  con- 


stitute  micaceous  iron  ore.  Both,  when  rubbed  down, 
furnish  a  dark  red  powder  of  no  particular  beauty. 
Micaceous  iron  ore  forms  the  transition  stage  into 
frothy  hematite,  or  iron  cream,  the  sole  difference 
being  that  the  crystals  of  the  latter  are  much  smaller, 
and  the  scales  finer,  the  iron-black  colour  passing 
gradually  into  cherry  red.  At  the  same  time,  the 
lustre,  though  still  high,  loses  most  of  the  metallic 
sheen  exhibited  by  micaceous  iron  ore. 


The  variety  known  as  hematite  or  bloodstone, 
sometimes  occurring  as  shiny  nodules,  is  distinguished 
by  its  handsome  red  colour.  Some  of  the  lumps  are 
composed  of  long,  thin  crystals  grouped  about  a 
common  centre  so  as  to  form  a  globular  mass.  Despite 
its  bright  colour,  the  hardness  of  hematite  (between 
3  and  5)  prevents  it  from  being  used  as  a  pigment, 
the  value  of  the  product  not  being  commensurate 
with  the  cost  of  reduction. 


There  are  numerous  deposits  of  red  ironstone,  in 
the  state  of  fine  earth,  where  the  operations  of  grinding 
and  levigation  have,  to  a  considerable  degree,  already 
been  carried  out  by  Nature.  These  deposits  form  the 
mineral  which,  under  the  name  of  raddle,  is  often  used 
as  a  pigment  for  ordinary  paints.  It  may  be  con- 
sidered to  have  originated  in  the  transformation  of 
red  ironstone,  by  the  natural  forces  that  can  every- 
where be  seen  disintegrating  rocks,  namely  water 
and  frost,  into  a  fine  powder,  which  has  been  trans- 


ported,  often  over  long  distances,  by  water,  and  has 
finally  settled  down. 

In  places  where  the  process  has  been  carried  out  in 
this  manner,  the  raddle  will  be  in  a  condition,  as  regards 
fineness  of  division  and  beauty  of  colour,  that  leaves 
nothing  to  be  desired,  and  the  material  itself  is  ready 
for  use  as  a  very  valuable  pigment.  Large  deposits 
of  this  kind,  however,  are  of  rare  occurrence;  but 
there  are  plenty  in  which  the  ferric  oxide  is  associated 
with  varying  quantities  of  clay,  sand,  and  sometimes 

The  conditions  here  are  on  all  fours  with  those  of 
clay,  which,  too,  has  been  formed  in  a  similar  way. 
Pure  clay,  the  so-called  kaolin,  is  a  highly  valuable 
material,  whereas  ordinary  loam — highly  contaminated 
clay — is  only  of  low  value.  In  judging  the  quality  of 
raddle  as  a  pigment,  the  presence  of  impurities  is  of 
less  account  than  their  nature;  and  in  some  cases  a 
very  highly  contaminated  raddle  may  be  worth  far- 
more,  as  a  pigment,  than  one  containing  only  very 
small  admixtures  of  extraneous  substances. 

As  stated  above,  the  ordinary  impurities  in  raddle 
are  clay,  lime  and  quartz  sand.  An  admixture  of 
clay,  even  if  fairly  large,  is  no  great  drawback,  since 
the  material  can  be  used  in  its  natural  state,  and  also 
be  toned  by  burning.  Lime  is  less  favourable,  for 
though  a  calcareous  raddle  can  be  used  as  it  is,  the 
lime  parts  with  its  carbon  dioxide  on  calcination, 
becoming  changed  into  caustic  lime  and  imparting 
to  the  product  qualities  which  preclude  its  employment 
for  a  number  of  purposes,  especially  for  mixing  with 
delicate  organic  colours. 

The  presence  of  quartz  sand  is  immaterial  when  the 
raddle  is  to  be  burned,  inasmuch  as  sand  is  unaltered 


by  calcination.  But  it  constitutes  a  drawback  because 
it  makes  the  fine  raddle  gritty  and  unsuitable  for  fine 
paint  work.  The  only  way  to  eliminate  this  impurity 
is  by  levigation — an  expensive  operation  which  should, 
as  far  as  possible,  be  avoided  for  these  native  ferric 
oxides,  because  they  must  be  sold  very  cheaply,  and 
have  to  compete  with  the  large  quantities  of  oxide 
obtained  as  a  by-product  of  the  chemical  industry. 

The  suitability  of  a  given  specimen  of  raddle  for  use 
as  a  pigment  may  be  easily  ascertained  by  weighing 
out  exactly  100  grams  and  heating  to  about  120°  C. 
The  loss  of  weight  will  give  the  amount  of  water  in 
mechanical  retention.  The  residue  is  suffused  with 
strong  vinegar,  and  left  for  several  days,  being  stirred 
at  frequent  intervals.  The  carbonates  of  lime  and 
magnesia  present  will  dissolve  in  the  acid,  the  ferric 
oxide  remaining  untouched.  The  liquid  is  decanted, 
and  the  residue  washed  several  times  writh  water  and 
dried,  the  diminution  in  weight  being  a  measure  of 
the  carbonates  in  the  sample.  If  the  vinegar  has 
turned  a  yellow  colour,  the  presence  of  ferric  hydroxide 
in  the  mineral  is  indicated,  this  hydroxide  being  readily 
soluble  in  acetic  acid.  If  the  residue  feels  gritty,  it 
contains  quartz  sand,  the  amount  of  which  can  be 
found  with  sufficient  accuracy  by  levigating  the  mass 
and  weighing  the  sandy  residue  after  drying. 

Deposits  occur,  in  many  places,  of  a  mineral  similar 
to  raddle,  but  formed  under  peculiar  conditions. 
Thus,  there  are  found,  in  the  vicinity  of  brown-coal 
deposits  that  are  rich  in  pyrites,  earthy  masses  which 
are  occasionally  of  a  handsome  red  colour  and  consist 
of  a  variety  of  minerals  admixed  with  a  considerable 
proportion  of  ferric  oxide. 

These  masses  probably  originated  in  fires  in  the  coal 


seams,  whereby  the  pyrites  became  transformed  into 
ferric  oxide  and  basic  ferric  sulphate;  and  where  the 
deposits  are  of  sufficient  size,  they  may  be  advan- 
tageously utilised  in  the  production  of  cheap  reds. 
In  most  cases,  however,  the  minerals  must  be  levigated, 
owing  to  the  frequency  with  which  they  contain  large 
proportions  of  extraneous  minerals  in  a  gritty  condition. 


It  has  already  been  stated,  in  dealing  with  the 
yellow  ochres,  that  these  colours  can  be  toned  by 
burning,  part  of  the  ferric  hydroxide  losing  its  water 
and  changing  into  red  ferric  oxide.  The  more  severe 
the  burning,  the  larger  the  amount  of  ferric  oxide 
formed  and  the  nearer  the  colour  of  the  product 
approximates  to  red.  According,  however,  as  the 
original  cchre  was  yellow  or  brown,  the  tone  of  the  burnt 
colour  will  lie  between  orange  and  brownish  red.  If 
the  heating  be  pushed  so  far  as  to  transform  all  the 
ferric  hydroxide  into  oxide,  the  red  will  come  more 
and  more  into  prominence  in  proportion  to  the  amount 
of  hydroxide  in  the  original  material.  If  the  product 
consists  entirely  of  ferric  oxide,  as  is  the  case  with  that 
obtained,  as  a  by-product,  in  the  manufacture  of  English 
sulphuric  acid,  a  pure  red  ferric  oxide  (caput  mortuum, 
colcothar,  English  red,  etc.)  will  be  obtained.  If  the 
heating  be  increased  above  a  certain  point,  the  pure 
ferric  oxide  will  change  colour,  assuming  a  brown  to 
violet  tone  according  to  the  temperature  employed. 

(a)  Burning  in  the  Muffle 

Since,  as  a  rule,  the  quantity  of  material  treated 
in  the  preparation  of  these  brown,  violet  to  black  ferric 



oxide  pigments  for  the  purposes  of  the  painter  on 
porcelain  is  not  large,  the  same  kind  of  muffle  furnace 
(Fig.  29)  as  serves  for  making  enamels  can  be  used. 
The  fireclay  muffle  M  is  inserted  in  a  reverberatory 
furnace  0,  with  a  good  draught,  and  is  raised  to  a 
white  heat.  The  finely  powdered  material  to  be  burned 
is  spread  out  evenly  on  plates  of  sheet-iron  or  fire-clay, 

FIG.  29. 

and  introduced  into  the  white-hot  muffle,  where  it  is 
left  for  a  period  corresponding  to  the  colour  desired. 
To  save  time,  the  plates  may  be  pre-heated  in  a  second 
muffle  arranged  above  the  first. 

By  this  means  a  large  range  of  tones  can  be  obtained 
from  one  and  the  same  material,  by  heating  it  to 
different  temperatures;  and  the  colours,  so  produced 
are  distinguished,  not  only  by  their  warmth  of  tone, 
but  also  by  very  high  stability.  In  fact,  they  may  be 


regarded  as  permanent,  because  very  strongly  calcined 
ferric  oxide  only  passes  very  slowly  into  solution  even 
under  prolonged  boiling  in  the  strongest  acids.  Owing 
to  this  excellent  property,  which  is  equalled  by  very 
few  other  pigments,  and  the  low  cost  of  preparation, 
these  colours  deserve  the  most  careful  consideration 
by  all  manufacturers  who  are  in  a  position  to  obtain 
suitable  material  in  sufficient  quantities. 

(b)  Caput  Mortuum,  Colcothar 

Previous  to  the  English  method  of  making  sulphuric 
acid  by  the  oxidation  of  sulphur  dioxide  with  nitric 
acid,  this  acid  was  manufactured  by  heating  dehydrated 
ferrous  sulphate  (green  vitriol) ;  and  even  now,  fuming 
sulphuric  acid — oil  of  vitriol,  or  Nordhausen  sulphuric 
acid — is  largely  obtained  by  the  same  process. 

When  anhydrous  ferrous  sulphate,  FeS04,  is  exposed 
to  a  very  high  temperature— strong  white  heat — it  is 
decomposed  into  sulphur  trioxide,  SO3,  sulphur  dioxide, 
SO2,  and  a  residue,  mainly  composed  of  ferric  oxide 
and  a  little  basic  ferric  sulphate,  which  remains  behind 
in  the  heating-pan.  In  fact,  even  at  the  highest 
possible  temperatures  obtainable  in  the  furnaces  used 
for  the  distillation  of  the  green  vitriol,  it  is  impossible 
to  recover  the  whole  of  the  sulphuric  acid,  a  small 
portion  being  tenaciously  retained  by  the  iron. 

This  red  residue  is  sold  under  various  names — 
colcothar,  caput  mortuum,  English  red,  Indian  red, 
etc. — and  is  used  as  a  low-grade  pigment,  and  also  as  a 
polishing  agent.  The  name  caput  mortuum  is  a 
survival  from  the  time  of  the  alchemists,  and  was 
probably  applied  to  indicate  a  dead-burned  product, 
from  which  all  the  active  ingredients  had  been  removed. 

Although,  in  former  ages,  this  substance  was  held 


in  low  estimation  as  a  pigment,  attempts  have  been 
made  in  recent  times  to  convert  it,  by  suitable  treat- 
ment, into  a  more  valuable  product ;  and  these 
attempts  have  been  crowned  with  success,  affording 
another  instance  of  how  a  high  commercial  value  can 
be  imparted  to  a  waste  product  by  proper  manipulation. 

(c)  Calcining  Ferric  Oxide 

In  order  to  obtain  a  series  of  tones  of  colcothar,  it 
is  subjected  to  repeated  calcination,  but  not  by  itself, 
since  it  would  require  an  extremely  large  quantity  of 
fuel  to  effect  any  change  of  tone  in  view  of  the  very 
high  temperature  the  material  has  already  been 
exposed  to  in  the  sulphuric  acid  plant.  If,  however, 
salt  be  added,  then  a  variety  of  tones  can  be  obtained 
without  recourse  to  any  particularly  high  temperature. 
It  is  frequently  stated  that  the  only  effect  of  the 
presence  of  salt  is  to  keep  the  calcining  temperature 
uniform,  inasmuch  as  the  salt  volatilises  at  a  strong 
red  heat,  and  when  that  temperature  is  reached,  the 
whole  mass  cannot  get  any  hotter  until  the  whole  of 
the  salt  has  passed  off,  all  the  heat  applied  being 
consumed  in  transforming  the  salt  into  the  state  of 

As  a  rule,  however,  the  amount  of  salt  added  does 
not  exceed  6%  of  the  weight  of  the  charge  to  be 
calcined ;  and  this  quantity  does  not  seem  to  be 
sufficient  to  keep  the  temperature  at  a  uniform  level 
through  the  several  hours  required  for  the  calcining 
process.  The  author  is  therefore  of  opinion  that  the 
salt  also  has  a  chemical  action  on  the  material  during 
the  calcination. 

As  already  mentioned,  colcothar  is  by  no  means 
pure  ferric  oxide,  but  always  contains  basic  ferric 


sulphate.  Now,  it  is  feasible  that  some  reaction  may 
take  place  between  the  basic  sulphate  and  the  sodium 
chloride  at  calcination  temperature,  with  the  formation 
of  caustic  soda,  which,  being  a  far  more  powerful  base 
than  ferric  oxide,  deprives  the  latter  of  sulphuric  acid, 
sodium  sulphate  being  formed.  The  chlorine  of  the 
salt  combines  with  the  iron  to  form  ferric  chloride, 
which  volatilises  at  a  glowing  heat. 

According  to  this  hypothesis,  therefore,  the  addition 
of  common  salt  in  the  calcination  of  colcothar  is  less 
for  the  purpose  of  maintaining  a  uniform  temperature 
within  certain  limits  than  for  decomposing  the  basic 
ferric  sulphate  present  and  inducing  the  formation  of 
a  product  consisting  entirely  of  pure  ferric  oxide. 
The  various  tones  obtained  are  due  to  the  varying 
length  of  exposure  to  the  heat. 

The  following  method  is  pursued  in  the  conversion 
of  colcothar  into  iron  pigments  on  a  manufacturing 
scale.  The  crude  colcothar  from  the  sulphuric  acid 
j)lant  is  ground,  as  finely  as  possible,  in  ordinary  mills, 
and  the  resulting  soft  powder  is  intimately  mixed  with 
salt,  2,  4  or  6%  being  the  usual  proportions  added. 
The  calcination  is  ordinarily  continued  for  six  hours  in 
the  case  of  the  mixture  containing  the  largest  amount 
of  salt ;  but  only  two  hours,  or  even  one,  for  the  other 

The  operation  is  carried  on  in  earthenware  pipes,  a 
large  number  of  which  (up  to  sixty)  are  built  into  a 
furnace.  The  latter  is  fired  very  carefully,  the  tem- 
perature being  raised  only  very  gradually,  since  ex- 
perience has  shown  that  much  better  coloured  products 
are  obtained  in  this  way  than  by  raising  the  mass 
quickly  to  a  high  temperature. 

When  incandescent  ferric  oxide  is  allowed  to  cool 


down  with  unrestricted  access  of  air,  the  colour  is  not 
nearly  so  bright  as  when  air  is  excluded  during  the 
cooling.  Since  air  has  no  action  on  ferric  oxide,  this 
remarkable  phenomenon  cannot  be  due  to  the  presence 
of  the  air,  but  probably  to  the  influence  exerted  by  the 
rapid  change  of  temperature  on  the  arrangement  of 
the  finest  particles  of  the  oxide.  Nevertheless,  some 
manufacturers  hold  that  rapid  cooling,  with  restricted 
access  of  air,  improves  the  colour. 

To  exclude  air  from  the  ferric  oxide  during  calcina- 
tion, the  open  ends  of  the  pipes  are  flanged  and  covered 
with  close-fitting  plates,  which  are  luted  with  clay. 
The  expansion  of  the  internal  air  as  it  grows  hot  would 
burst  the  pipes  unless  a  means  of  escape  were  provided, 
which  consists  in  leaving  small  vent  holes  in  the  cover 

As  previously  mentioned,  calcined  ferric  oxide  is 
very  inert,  chemically,  so  that,  when  the  calcination 
has  been  strong,  prolonged  boiling  with  the  most  power- 
ful acids  is  needed  to  bring  the  oxide  into  solution. 
If  the  heating  has  been  continued  up  to  the  strongest 
white  heat,  and  the  ferric  oxide  maintained  in  that 
condition  for  several  hours,  even  hot  sulphuric  acid 
will  have  only  a  slight  effect  on  the  oxide,  and  the  only 
way  to  make  it  more  readily  soluble  is  by  fusion  with 
potassium  bisulphate. 

Now  indifference  to  chemical  action  is  just  the 
property  required  of  a  pigment  for  fine  work;  and  in 
this  respect,  the  ferric  oxide  colours  are  superior  to  all 
others.  The  gradations  of  tone  that  can  be  obtained 
from  ferric  oxide  by  varying  the  calcination  are  very 
numerous,  comprising  all  between  iron  red,  red-brov/n 
and  pure  violet. 

The   author  has   tried   heating   ferric   oxide   for   a 


considerable  time  at  a  very  high  temperature,  equiva- 
lent to  the  strongest  white  heat,  and  obtained  a  product 
which  was  no  longer  pure  violet,  but  had  a  decidedly 
blackish  colour.  Perhaps,  by  greatly  prolonging  the 
heating,  it  might  be  possible  to  get  a  pure  black;  but, 
even  if  this  were  so,  the  matter  would  be  of  no  special 
interest,  because  black  pigments  for  paints  can  be 
prepared  in  a  much  cheaper  manner.  All  that  would 
be  accomplished  would  be  the  proof  that  ferric  oxide 
actually  undergoes  an  extensive  molecular  modification 
when  heated. 


Alum  is  manufactured  from  alum  shale  and  alum 
earth,  the  former  being  a  carbonaceous  clay  shale 
interspersed  with  pyrites,  and  the  latter  a  clay  charged 
with  pyrites  and  bitumen.  The  raw  materials  are  left 
in  heaps  for  several  years,  the  pyrites  being  thereby 
oxidised  with  formation  of  free  sulphuric  acid  and 
ferrous  sulphate.  This  free  acid  reacts  further  on  the 
clay,  which  it  transforms  into  sulphate  of  alumina; 
and  by  leaching  the  heaps  with  water,  a  solution  is 
obtained  which  contains  the  sulphate  of  alumina  and 
the  ferrous  sulphate.  On  the  liquor  being  concen- 
trated, a  basic  ferric  sulphate  is  deposited,  which  is 
worked  up  into  red  pigment. 

For  this  purpose  it  is  first  levigated  in  a  special 
manner,  the  sludge  from  the  pans  being  placed  in  a 
large  vat,  suffused  with  water,  and  kept  in  slow  circula- 
tion by  stirrers,  which  distribute  the  particles  in  the 
water,  forming  a  turbid  liquid.  This  liquid  is  con- 
ducted into  a.  gently  sloping  shute,  the  sides  of  which 
are  perforated  with  openings  at  certain  intervals,  to 



allow  part  of  the  liquid  to  run  off  into  large  collecting 
vessels  underneath. 

The  heaviest  of  the  suspended  particles  settle  down 
first  and  are  flushed  out  by  the  water  escaping  through 
the  first  opening.  The  finer  the  particles,  the  longer 
they  remain  in  suspension,  so  that  the  liquid  escaping 
through  the  last  holes  carries  off  only  a  very  fine 
powder.  The  liquid  collected  in  the  different  vessels 
is  allowed  to  subside  and  is  then  drawn  off  from  the 

FIG.  30. 

firm  deposit.  The  operation  is  repeated  with  fresh 
quantities  of  sludge  until  sufficient  sludge  has  been 
collected  for  further  treatment.  The  collecting  vessel 
furthest  away  from  the  intake  of  the  shute  contains 
the  finest  levigated  material,  and  this  is  used  for  making 
the  best  ochres. 

The  levigated  mass  is  dried  in  a  very  simple  manner, 
being  usually  spread  out  on  boards,  which  are  exposed 
to  the  air  in  open  sheds,  covered  with  a  roof  to  keep 
out  the  rain.  Here  the  sludge  is  left  until  it  forms  a 
pasty  or  earthy  mass,  and  is  then  calcined. 



The  best  calcining  furnace  is  of  the  type  used  for 
colcothar;  but  the  pipes  must  be  connected  to  an 
exhaust  pipe  for  carrying  off  the  vapours  disengaged 
during  calcination. 

However,  since  alum  manufacturers  do  not  usually 
go  in  for  making  the  highest-grade  pigments,  simpler 
calcining  furnaces  are  used,  consisting  of  reverberatory 
furnaces  in  which  the  heating-gases  are  allowed  to  act 
directly  on  the  materials  of  the  charge.  A  front 

FIG.  31. 

elevation  and  section  of  such  a  furnace  are  shown  in 
Figs.  30  and  31.  The  furnace  is  constructed  with 
several  arches,  one  above  another,  marked  c,  k,  d. 
The  charge  is  introduced  through  the  openings  b  and 
b'.  The  furnace  chamber  is  at  a,  and  the  ashpit  at  g. 
The  gases  of  combustion  flow  over  the  charge  on  the 
hearths  of  the  several  arches  and  escape,  at  the  top, 
into  the  stack,  along  with  the  acid  vapours  liberated 
from  the  glowing  mass. 

The  further  the  hot  gases  get  away  from  the  fire, 
the  cooler  they  become,  and  therefore  the  less  strongly 
heated  the  charge  on  the  upper  hearths.  Consequently, 


the  resulting  product  (ferric  oxide)  from  the  different 
stages  of  the  furnace  differs  in  colour;  and  a  number 
of  gradations  can  be  obtained  by  blending.  The  ferric 
oxide  pigments  prepared  in  this  way  are  not  pure 
oxide,  but  also  contain  small  quantities  of  sulphuric 
acid  and  metallic  oxides  which  were  present  in  the 
original  crude  sludge.  However,  by  reason  of  the 
simple  process  of  preparation  employed,  these  pigments 
are  usually  sojd  at  lower  prices  than  those  from  colco- 
thar;  and  for  less  fine  work  they  are  excellent. 



IN  point  of  chemical  composition,  the  majority  of 
the  brown  earth  colours  are  closely  allied  to  the  reds, 
both  kinds  containing  ferric  oxide.  The  main  difference 
consists  in  that,  in  the  brown  earths,  the  ferric  oxide 
is  combined  with  water  to  form  ferric  hydroxide. 

Many  of  the  brown  earth  colours,  however,  are  of 
entirely  different  chemical  composition,  and  either 
consist  mainly  of  organic  matter  derived  from  the 
decomposition  of  plants — and  therefore  very  similar 
to  brown-coal  or  peat — or  else  contain  varying  quanti- 
ties of  inorganic  substances  mixed  with  these  dark- 
coloured  organic  decomposition  products. 

The  brown  earth  colours  form  a  highly  important 
group,  some  of  the  members  of  which  are  used  in  the 
finest  paintings,  and,  for  certain  purposes,  cannot  be 
replaced  by  other  pigments.  Those  containing  ferric 
hydroxide  are  found — though  not  very  frequently — in 
natural  deposits,  the  most  celebrated  being  the  terra 
di. Siena,  occurring  in  the  vicinity  of  that  city. 


This  highly  renowned  pigmentary  earth  is  found  in 
deposits,  and,  in  the  crude  state,  forms  dark  brown 
masses  which  are  devoid  of  lustre,  crumble  readily 
between  the  fingers,  have  a  smooth  conchoidal  fracture 


and  absorb  water  with  avidity,  in  consequence  of  which 
property  they  adhere  to  the  tongue.  Their  chief 
chemical  constituent  is  ferric  hydroxide,  with  which, 
however,  variable  quantities  of  sand,  clay  and  ferric 
oxide  are  admixed.  These  admixtures  cause  a  con- 
siderable divergence  in  the  colour  of  the  earth,  ranging 
from  pure  brown  to  reddish -brown,  and,  in  the  case 
of  very  impure  lumps,  to  an  unsightly  yellow-brown. 

Mineralogically,  terra  di  Siena  is  often  regarded  as  a 
distinct  species  which,  according  to  the  results  of 
analysis,  must  be  considered,  not  as  ferric  hydroxide, 
but  as  ferric  silicate  combined  with  water.  Sometimes, 
a  portion  of  the  ferric  oxide  is  replaced  by  alumina,  so 
that  the  percentage  composition  of  the  mineral  becomes 
approximately:  ferric  oxide,  66%;  silica,  n%; 
alumina,  10% ;  and  water,  13%.  The  hardness  of  this 
mineral  is  2*5,  and  the  sp.  gr.  3-46. 

The  method  of  formation  of  terra  di  Siena  was 
probably  on  the  same  lines  as  that  already  described 
in  the  case  of  ochre,  namely  by  the  breaking  down  of 
minerals — in  this  case  brown  ironstone — and  natural 
levigation,  the  powder  being  deposited  in  places  where 
the  water  containing  the  ferric  hydroxide  in  suspension 
came  to  rest  and  allowed  the  solid  particles  to  settle 

The  best  lumps  of  terra  di  Siena  in  point  of  purity 
and  colour  can  be  used  as  pigments  without  any 
preparation;  but  in  most  cases  the  earth  is  lightly 
calcined,  in  order  to  improve  the  colour.  This  treat- 
ment enables  a  whole  series  of  tones,  from  pure  brown 
to  the  brightest  red,  to  be  obtained.  The  stronger  the 
heating,  the  more  water  expelled  from  the  hydroxide, 
and  consequently  the  closer  the  approximation  of  the 
colour  to  that  of  ferric  oxide. 


The  pigments  met  with  in  commerce  as  terra  di 
Siena  can  also  be  prepared  artificially,  by  making  ferric 
hydroxide  and  heating  this,  when  dried,  until  the 
requisite  tone  is  attained.  For  this  purpose,  ferrous 
oxide  is  precipitated  from  green  vitriol  and  exposed  to 
the  air,  under  which  conditions  it  is  rapidly  transformed 
into  ferric  oxide,  and  the  greyish-green  colour  of  the 
mass  changes  to  brown.  Lighter  tones  can  be  obtained 
by  the  addition  of  inert  white  substances;  and,  in 
other  respects,  the  method  of  preparation  is  the  same 
as  that  of  artificial  ochre. 

These  pigments  are  sold  under  various  names,  the 
dark  shades,  between  pure  brown  and  red  brown,  being 
usually  called  terra  di  Siena  or  mahogany  brown, 
whilst  the  paler  sorts  are  sold  as  satinober — more 
correctly  satin  ochre,  golden  ochre,  etc.  Other  pig- 
ments, chemically  allied  to  the  ferric  oxide  or  ochre 
pigments,  are  sometimes  found  on  the  market  under 
various  and  entirely  arbitrary  names. 

It  may  be  pointed  out  that  the  greatest  confusion 
exists  in  the  nomenclature  of  pigments,  to  such  an 
extent  that,  in  many  cases,  neither  the  chemist  nor 
the  manufacturer  knows  precisely  what  pigment  is 
implied  by  a  given  name.  The  confusion  is  still  further 
increased  by  the  use  of  names  taken  from  different 


Umber,  properly  so  called — also  known  as  Turkish, 
Cyprian  or  Sicilian  umber,  from  the  country  of  origin 
• — derives  its  name,  according  to  some  authorities,  from 
the  province  of  Umbria  (Italy),  where  a  brown  earth 
is  found,  though  others  ascribe  it  to  the  Latin  "  umbra  " 


(shade)  because  of  the  pigment  being  used  for  painting 

True  umber  is  an  earthy  mass  of  fine  texture  and 
liver-brown  colour,  merging  into  chestnut  in  some  of 
the  lumps.  Chemically,  it  consists  of  a  double  silicate 
of  iron  and  manganese  combined  with  water,  a  portion 
of  these  metals  being  usually  replaced  by  alumina. 
The  greater  hardness  (1*5)  and  higher  specific  gravity 
(2*2)  of  true  umber  in  comparison  with  Cologne  earth 
(which  is  quite  arbitrarily  termed  "umber-"),  form  a 
ready  means  of  differentiation  between  the  two. 

According  to  Viktor  Merz,  the  umber  found  in 
Cyprus  consists  of  :  ferric  oxide,  52% ;  manganese 
oxide,  14*5%;  and  alumina,  3%;  and  is,  possibly, 
merely  a  mixture  of  clay  with  hydroxide  of  iron  or 
manganese.  An  umber  examined  by  Klaproth  con- 
tained 13%  of  silica,  5%  of  alumina,  48%  of  ferric 
oxide,  20%  of  manganese  oxide  and  14%  of  water. 

The  tone  of  umber  can  be  modified,  in  the  direction 
of  red,  by  calcination,  but  this  process  is  seldom 
employed,  the  dark  brown  shade  of  this  colour  being 
the  one  most  appreciated. 

In  some  parts  of  northern  German}7,  Thuringia  in 
particular,  the  iron  mines  contain  smaller  or  larger 
pockets  of  ferric  hydroxide,  of  a  fine  earthy  texture, 
from  which  umber  is  prepared,  by  levigation  and 
calcination.  The  product  is  sold  under  various  names  : 
chestnut  brown,  wood  brown,  mahogany  brown,  bistre 
flea  brown,  roe  brown,  according  to  the  shade  of  the 
calcined  pigment. 

A  mineral  ("  siderosilicate,"  according  to  Von  Walter- 
hausen)  composed  of  ferric  silicate,  and  approximating 
in  this  respect  to  terra  di  Siena,  is  found  in  the  neighbour- 
hood of  Passaro  (Sicily)  in  deposits  of  tuff.  It  forms 


masses  which  are  transparent  at  the  edges  and  are 
usually  liver-brown  to  chestnut  in  colour.  The  hard- 
ness of  the  mineral  is  2*5,  the  sp.  gr.  2*713,  and  the 
average  chemical  composition:  silica,  34%;  ferric 
oxide,  48-5%  ;  alumina,  7-5%  ;  and  water,  10%. 

The  foregoing  are  only  a  few  examples  of  brown  or 
red-brown  earth  colours.  In  all  these  minerals  the 
pigmentary  principle  is  iron,  in  combination  either  with 
oxygen  alone  (ferric  oxide),  with  oxygen  and  water 
(ferric  hydroxide),  or  silica  compounds  (ferric  silicate), 
and  always  associated  with  certain  quantities  of  other 
metallic  oxides,  especially  alumina  and  manganese 
oxide.  Although  but  few  of  these  minerals  have 
gained  any  special  reputation  as  pigments,  there  is 
no  doubt  that  similar  minerals,  which  are  certain  to 
occur  in  or  near  many  deposits  of  iron ,  ores,  could 
equally  well  be  used  for  that  purpose.  There  is  no  need 
to  emphasise  that  the  discovery  of  such  a  mineral 
would  be  a  very  valuable  find,  and  that  the  products 
obtainable  therefrom  could  be  utilised  to  great 

The  testing  of  a  mineral  for  its  suitability  as  pigment 
is  a  very  simple  matter,  all  that  is  required  being  to 
subject  a  small  quantity  to  the  same  treatment  that 
is  applied  to  the  earth  colours  on  a  large  scale.  For 
this  purpose  a  few  pounds  of  the  mineral  are  levigated, 
and  the  residue  is  dried.  To  ascertain  the  tones 
obtainable  by  calcination,  small  samples — of  about 
100  grms. — are  placed  in  crucibles,  and  gradually 
heated  in  a  furnace.  When  the  masses  have  attained 
a  sufficient  temperature,  the  samples  are  taken  out  of 
the  furnace,  at  intervals  of  ten  minutes,  and  left  to 
cool.  It  will  then  not  be  difficult  to  decide  whether 
the  mineral  is  at  all  suitable  for  the  purposes  of  the 


colour-maker;  and  if  so,  these  tests  afford  at  once  an 
indication  of  the  temperature  and  time  the  mineral 
must  be  heated  in  order  to  obtain  pigments  of  definite 


The  application  of  the  term  "  umber  "  to  this  earth 
can  only  have  been  based  on  a  certain  similarity  in 
colour  to  true  umber.  In  chemical  composition,  how- 
ever, the  two  are  quite  different,  Cologne  earth  really 
consisting  of  a  mixture  of  humic  substances.  It  is 
well  known  that  the  rotted  wood  found  in  the  interior 
of  decaying  trees  is  often  a  handsome  brown  colour; 
and  all  woody  matter,  after  lying  a  very  long  time, 
finally  acquires  this  colour,  owing  to  the  transformation 
of  the  wood  into  dark-coloured  compounds  richer  in 
carbon.  This  effect  can  be  seen  on  the  large  scale,  in 
Nature,  in  the  case  of  coal,  brown  coal  and  peat. 

Now  Cologne  earth  consists  of  a  brown-coal  mould, 
dark  brown  in  colour,  of  earthy  character  and  of  such 
low  cohesive  power  that  it  crumbles  with  ease.  Owing 
to  this  character,  Cologne  earth  can  be  easily  ignited 
by  the  flame  of  a  candle,  and  then  burns  with  a  strong, 
smoky  flame,  leaving  very  little  ash  and  disseminating 
the  peculiar  bituminous  smell  given  off  when  brown 
coal  is  burned. 

The  geological  characteristics  of  Cologne  earth  enable 
one  to  conclude  that,  where  similar  conditions  prevail, 
materials  of  analogous  nature  may  be  discovered.  This 
earth  is  found  embedded  in  a  deposit  of  brown  coal,  in 
which  it  forms  pockets,  and  occasionally  large  bodies. 
Now,  brown-coal  deposits  of  enormous  extent  occur  in 
very  many  localities,  as  for  instance  in  Upper  Austria 


and  in  Bohemia;  and  many  of  these  mines  are  sure 
to  contain  pockets  of  brown-coal  mould,  which  have 
perhaps  been  overlooked,  but  might  very  well  be 
utilised  in  the  preparation  of  colours  of  very  similar 
character  to  Cologne  earth. 

The  preparation  of  this  material  is  very  simple. 
The  earth  coming  from  the  deposits  is  put  through  a 
simple  levigation  treatment  which  leaves,  as  residue, 
lumps  of  semi-decomposed  wood,  mineral  admixtures, 
sand,  etc.  The  levigated  earth  is  sold  in  the  form  of 

Cologne  earth  comes  into  the  market  under  various 
other  names,  such  as  :  umber,  Cassel  brown.  Spanish 
brown,  etc. 

The  fiery  brown  which  was  so  greatly  preferred  by 
the  famous  painter  Van  Dyck,  and  named  Vandyke 
brown  after  him,  was  of  very  similar  composition  to 
Cologne  earth,  and  is  said  to  have  been  obtained  from 
a  deep  brown  peat  earth.  The  Vandyke  brown  of  the 
present  day,  however,  is  almost  invariably  a  ferric 
oxide  pigment,  toned  to  the  proper  shade  by  suitable 


As  a  natural  product,  which  can  be  used  as  a  painters' 
colour  without  any  special  preparation,  asphaltum 
(bistre,  bitumen)  may  also  be  classed  among  the  earth 
colours.  Chemically,  it  is  composed  of  hydrocarbons 
of  various  kinds,  and  is  thus  similar  to  tar;  in  fact, 
asphaltum  may  also  be  regarded  as  a  natural  tar 
resulting  from  the  decomposition  of  various  orgaric 
substances.  Many  deposits  of  this  mineral  are  known, 
and  two  of  them  are  particularly  celebrated  :  those 


of  the  Dead  Sea,  in  Syria,  and  the  Lake  of  Asphalt,  in 
Trinidad.  Both  deposits  consist  of  craters  filled  with 
water  on  which  the  asphaltum  floats  in  large  cakes. 

Several  kinds  of  asphaltum  are  met  with  in  com- 
merce, ranging  in  colour  from  brown  to  black.  The 
preparation  of  the  material  as  a  pigment  is  confined  to 
grinding  the  mass,  which  is  always  of  a  low  degree  of 
hardness.  Being  readily  soluble  in  oil  of  turpentine 
and  then  furnishing  the  most  beautiful  brown  tones 
when  laid  on  thinly,  the  pigment  is  usually  sold  in  this 
condition,  although  it  is  also  ground  in  oil  for  the  same 

Finally,  it  may  be  mentioned  that  various  useless 
materials  can  be  transformed,  by  suitable  treatment, 
into  brown  pigments  closely  resembling  Cologne  earth 
and  applicable  to  the  same  uses.  Such  pigments  can 
be  prepared  from  brown-coal  slack  (from  inferior  brown 
coal)  or  bituminised  wood — a  variety  of  brown  coal 
looking  like  charred  wood — by  treating  these  materials 
with  a  lye  made  from  wood  ashes  and  lime,  and  washing 
and  drying  the  residue. 



ALTHOUGH  the  number  of  green-coloured  minerals 
is  large,  but  few  of  them  are  suitable  for  painters' 
colours,  because  they  occur  so  rarely  in  Nature  that 
their  employment  for  this  purpose  is  out  of  the  question, 
more  especially  since  a  very  large  number  of  green 
pigments  can  be  obtained  by  artificial  means.  The 
most  important  of  the  earth  colours  in  this  category 
are  Celadon  green,  or  green  earth,  and  malachite  green 
— the  latter,  however,  less  so,  because  the  substance  of 
which  it  is  composed  can  be  prepared  artificially. 


This  mineral  is  of  a  peculiar  green  colour,  and  the 
name  "  Celadon  green  "  has  been  universally  adopted 
in  the  nomenclature  of  colour  shades.  Green  earth 
occurs  native  in  many  places,  being  the  decomposition 
product  of  an  extensively  distributed  mineral,  augite, 
crystals  of  which  are  found  in  many  of  .the  deposits. 
The  green  earth  of  Monte  Valdo,  on  Lake  Garda  (Upper 
Italy)  has  been  used  for  a  very  long  time  as  a  pigment. 
It  is  chiefly  prepared  in  Verona  for  distribution  in 
commerce,  and  from  this  circumstance  has  acquired 
the  name  "  Verona  green,"  or  "  Verona  earth."  The 
earth  is  also  found  in  Cyprus  and  Bohemia,  where  it 




frequently  occurs  as  the  decomposition  product  of 
basaltic  tuff.  However,  whether  obtained  from  Monte 
Valdo  or  elsewhere,  the  product  is  always  placed  on 
the  market  as  Verona  earth. 

Native  green  earth  is  always  tough,  mostly  occurring 
in  amygdalous  lumps,  but  occasionally  in  the  crystalline 
form  of  augite.  It  has  a  fine-grained  fracture,  a  hard- 
ness between  i  and  2,  and  a  sp.  gr.  between  2*8  and  2*9. 
The  colour  is  not  always  quite  uniform,  pure  lumps 
having  the  characteristic  Celadon  green  appearance, 
whilst  impure  lumps  are  olive  green  to  blackish  green. 
In  chemical  composition  it  is  chiefly  ferrous  silicate, 
and  this  compound  must  be  regarded  as  the  actual 
pigmentary  principle  of  green  earth.  In  addition,  it 
contains  varying  quantities  of  other  compounds  which 
influence  the  depth  of  shade  of  the  product. 

Verona  earth  chiefly  consists  of  ferrous  oxide  in 
combination  with  silica;  alumina,  magnesia,  potash, 
soda  and  water  being  also  present.  Analysis  shows  it 
to  contain  :  ferrous  oxide,  21% ;  silica,  51% ;  magnesia, 
6%;  potash,  6%;  soda,  2%;  and  water,  7%. 

The  green  earths  from  Gosen,  Atschau  and  Mannels- 
dorf ,  near  Kaaden  (Bohemia)  and  the  Giant's  Causeway 
(Ireland)  have  the  following  composition  : 


^aden.1    ££^ 


4r          56-4 

Alumina   . 

3            2-1 

Ferrous  oxide 

23            5'i 

Ferric  oxide 




Magnesia  . 

2                 5'9 


3            8'8 

Carbon  dioxide 



—            6-8% 



On  account  of  the  large  quantity  of  mechanically 
associated  water,  freshly  dug  green  earth  is  greasy  in 
character,  like  wet  clay.  In  partial  drying,  most  of 
this  water  evaporates,  the  mass  becoming  earthy  and 
adherent  to  the  tongue.  Sometimes  the  colour  is  an 
ugly  brownish -green,  owing  to  the  presence  of  a  con- 
siderable amount  of  ferric  oxide  formed  as  the  result 
of  changes  set  up  by  exposing  the  mineral  to  the  air. 
Ferrous  oxide  is  a  very  unstable  compound,  having  an 
energetic  tendency  to  combine  with  more  oxygen  and 
thus  undergo  transformation  into  ferric  oxide ;  so  that 
when  green  earth  is  left  in  the  air  for  a  long  time,  a 
considerable  proportion  of  its  ferrous  oxide  is  oxidised 
to  ferric  oxide,  the  mass  thereby  assuming  the  brown 
tone  in  question. 

Such  an  unsightly  product  can,  however,  be  con- 
verted, by  simple  treatment,  into  one  of  very  bright 
and  handsome  appearance ;  and  it  is  this  possibility  that 
first  enabled  green  earth  to  attain  importance  as  a 
painters'  colour.  Formerly  it  was  only  used  as  a 
material  for  common  work,  being  added  to  whitewash 
or  employed  for  indoor  paints. 

When  the  crude  green  earth  is  treated  with  very 
diluted  hydrochloric  acid,  the  compound  of  ferrous 
oxide  and  silica  is  left  intact,  but  most  of  the  extraneous 
admixtures  are  removed.  Ferric  oxide,  in  particular, 
passes  into  solution,  and  the  calcium  carbonate  largely 
present  in  some  kinds  of  green  earth  is  also  dissolved. 
After  prolonged  contact  with  the  crude  earth,  the 
acid  liquor  takes  on  a  brownish  coloration  from  the 
dissolved  ferric  oxide.  Since  the  presence  of  iron 
salts  has  no  influence  on  the  purification  of  the  green 
earth,  the  most  impure,  highly  ferruginous  hydrochloric 
acid  can  be  used,  and  the  liquor  can  afterwards  be 


employed  in  the  preparation  of  artificial  ochre  by 
leaving  it  in  prolonged  contact  with  any  strongly 
ferruginous  mineral,  such  as  brown  ironstone,  which 
neutralises  the  surplus  acid.  This  liquor  is  then 
precipitated  by  lime,  alkali,  etc.,  the  resulting  deposit 
consisting  mainly  of  ferric  hydroxide,  the  further  treat- 
ment of  which  is  conducted  exactly  as  described  in 
dealing  with  the  preparation  of  artificial  ochre. 

The  treatment  of  the  crude  earth  is  best  carried  on 
in  the  same  vessels  as  are  to  be  used  in  the  subsequent 
levigation  process.  After  the  acid  liquor  has  been 
drawn  off,  the  earth  is  brought  into  contact  with  water, 
stirred  up  well,  and  the  wrater  run  off,  by  opening  tap- 
holes  in  the  side  of  the  vessel,  into  sett  ling- tanks,  where 
it  is  left  until  all  the  suspended  matter  has  completely 

The  colour  of  green  earth  can  also  be  toned  by  the 
addition  of  yellow  ochre,  thus  producing  a  range  of 
greens  with  a  yellowish  tinge.  These  lighter  shades, 
however,  are  seldom  met  with  in  commerce,  the  trade 
judging  the  quality  of  green  earth  more  particularly 
on  the  depth  of  colour. 

Green  earth  is  a  valuable  pigment  for  all  kinds  of 
painting,  on  account  of  its  extreme  permanence.  It 
may  be  applied  directly  over  lime  without  suffering 
any  change,  whereas  most  of  the  cheap  green  colours 
are  destroyed  in  like  circumstances.  This  behaviour 
renders  green  earth  specially  valuable  in  fresco  work, 
although  it  is  also  largely  used  as  an  oil  colour. 

Augite  is  of  frequent  occurrence  in  volcanic  districts ; 
and  in  such  localities,  deposits  of  green  earth  are  certain 
to  be  found.  The  test  for  the  suitability  of  a  green 
earth  consists  mainly  in  treatment  with  dilute  hydro- 
chloric acid.  If  the  mineral  assumes  a  handsome  green 


tone,  it  will  generally  form  a  useful  pigment.  The 
test  may  be  supplemented  by  applying  the  colour  to 
a  fresh  coating  of  whitewash,  under  which  conditions 
it  should  remain  unaltered. 


A  product  sometimes  put  on  the  market  as  green 
earth  or  green  ochre  has  nothing  beyond  its  name  in 
common  with  green  earth  properly  so  called,  except  a 
certain  similarity  in  colour.  This  pigment  is  prepared 
by  mixing  yellow  ochre  to  a  thin  pulp  with  water  and 
adding  about  2%  (of  the  weight  of  ochre)  of  hydro- 
chloric acid.  After  a  few  days,  a  solution  of  2  parts  of 
yellow  prussiate  of  potash  is  added,  and  if  the  liquor 
still  gives  a  precipitate  when  tested  with  ferrous 
sulphate,  this  last-named  salt  is  added  so  long  as  such 
a  precipitate  continues  to  form. 

The  deposit  is  washed,  and  dried  in  the  ordinary 
way.  When  the  right  proportions  have  been  taken,  a 
pigment  is  obtained  that  coincides  fairly  in  point  of 
tone  with  true  Verona  earth.  It  is,  however,  inferior 
in  point  of  permanence,  the  Berlin  blue  present  being 
somewhat  unstable  and  decomposing  very  quickly 
when  brought  into  contact  with  lime.  The  reaction 
taking  place  in  the  production  of  so-called  "  artificial 
Celadon  green  "  is  that  the  hydrochloric  acid  used 
dissolves  ferric  oxide  from  the  ochre,  the  addition  of 
the  yellow  prussiate  of  potash  then  forming  a  blue 
precipitate  of  Berlin  (Paris)  blue  which,  in,  conjunction 
with  the  yellow  of  the  ochre,  gives  a  green-coloured 



Although  the  pigment  sold  under  this  name  is  nearly 
always  an  artificial  product,  it  cannot  be  omitted  from 
a  work  dealing  with  the  earth  colours,  because,  in 
former  times,  it  was  prepared  exclusively  from  the 
mineral  malachite.  Owing  to  the  fact  that  artificial 
malachite  green  is  one  of  the  cheapest  of  colours,  the 
troublesome  work  involved  in  the  mechanical  prepara- 
tion of  the  native  pigment  has  been  almost  entirely 
abandoned,  and  the  malachite  itself  is  now  utilised  to 
greater  advantage  as  a  source  of  copper. 

Malachite  green  (or  mountain  green)  is  found  in  nearly 
every  case  where  copper  ores  exist,  and  is  still — though 
very  rarely  indeed — prepared,  in  a  few  places,  from  the 
mineral,  the  dark-coloured  lumps  being  picked  out 
because  the  lighter-coloured  ones  would  furnish  much 
too  pale  a  colour. 

The  treatment  of  malachite  for  the  preparation  of 
pigment  presents  certain  difficulties  owing  to  the  com- 
parative hardness  (3*5-4)  of  the  mineral,  which  is  also 
rather  heavy  (sp.  gr.  3*6-4-0).  On  the  large  scale,  the 
selected  mineral  is  first  put  through  a  stamping-mill, 
and  then  ground,  very  hard  stones  being  required  for 
this  purpose.  The  fine  product  from  this  (usually  wet) 
process  is  levigated  and  dried. 

The  pit  water  of  some  copper  mines  contains  certain 
quantities  of  blue  vitriol  (copper  sulphate)  in  solution  ; 
and  such  pit  water  is  generally  treated  for  the  recovery 
of  a  very  pure  form  of  copper,  the  so-called  cementation 
copper.  The  liquor  might  also  be  worked  up  into 
malachite  green,  by  collecting  it  in  large  tanks  and 
precipitating  the  dissolved  copper  oxide  with  milk  of 
lime,  the  bluish-green  deposit  separating  in  association 


with  gypsum  being  transformed  into  a  light  malachite 
green  by  washing  and  drying.  A  darker  green,  free 
from  gypsum,  could  be  prepared  by  using  a  solution  of 
carbonate  of  soda  as  precipitant. 

Neither  the  native  nor  the  artificial  malachite  green 
is  particularly  handsome  in  colour;  and  both  possess, 
in  addition,  the  unpleasant  property  of  gradually  going 
off  colour  in  the  air,  all  the  copper  compounds  being 
quite  as  sensitive  to  sulphuretted  hydrogen  as  those  of 
lead,  and  finally  turning  quite  black  under  the  influence 
of  that  gas. 



ONLY  three  minerals  are  known  to  be  suitable  as 
pigments ;  and  indeed,  at  present,  only  two,  the  third, 
lapis  lazuli,  being  now  of  merely  historical  interest. 
Nowadays,  no  one  would  think  of  using  this  rare  and 
expensive  mineral  as  a  pigment,  since  ultramarine, 
which  has  the  same  pigmentary  properties,,  is  extremely 
cheap,  whereas  the  pigment  from  lapis  lazuli  was 
worth  its  weight  in  gold.  The  only  two  blue  earth 
colours  of  any  interest  at  present  are  malachite  (copper) 
blue,  and  the  blue  iron  earth  Vivianite;  and  even 
these,  though  by  no  means  rare,  are  little  used,  since 
artificial  blues  are  now  made  which  are  far  superior 
in  beauty  and  can  be  obtained  so  cheaply  that  the 
natural  pigments  are  put  out  of  competition. 


Lazulite  and  malachite  (mountain  blue)  are  of 
frequent  occurrence  in  copper  mines,  and  the  former 
is  distinguished  by  its  beautiful  azure  blue  colour, 
which,  however,  suffers  considerably  when  the  crystals 
are  reduced  to  powder.  Both  minerals  are  very 
similar  in  chemical  composition,  and  consist  of  cupric 
carbonate.  The  formula  of  malachite  is  2CuOCO2  + 
H  O,  that  of  lazulite  being  3CuO(CO2)2  +  H2O,  so  that 



the  only  difference  between  them  is  that  of  the  relative 
proportions  of  the  substances  in  combination.  Lazulite 
is  also  rather  hard  (3-  5-4*0),  but  owing  to  the  small 
size  and  brittle  character  of  the  crystals  it  is  not  very 
difficult  to  pulverise.  In  the  air,  malachite  blue 
behaves  in  much  the  same  way  as  malachite  green, 
turning  black  in  presence  of  sulphuretted  hydrogen. 

Malachite  blue  is  chiefly  used  for  indoor  work,  and 
also  as  a  water  colour ;  but  it  is  always  rather  pale  and 


This  mineral — also  termed  blue  ochre — is  a  trans- 
formation product  of  various  iron  ores,  and  occurs 
native  as  fairly  extensive  deposits  in  some  places, 
especially  in  peat  bogs.  It  forms  ill-defined  crystals, 
which  are  of  a  low  degree  of  hardness  (2*0)  and  vary 
in  specific  gravity  between  2'6  and  2*7.  The  colour 
of  the  freshly  won  mineral  is  whitish  or  pale  blue,  but 
soon  changes  to  a  dark  blue  in  the  air,  owing  to  the 
oxidation  of  the  ferrous  phosphate,  originally  present, 
into  ferric  phosphate. 

Vivianite  can  be  transformed  into  a  pigment  by  a 
simple  process  of  crushing  and  levigation;  but  the 
product  is  never  very  handsome,  and,  at  best,  is  only 
suitable  for  quite  common  paint  work,  though  character- 
ised by  considerable  stability. 



ONLY  two  minerals  are  known  that  can  be  used  as 
black  earth  colours,  namely  black  chalk  or  shale  black, 
and  blacklead  or  graphite.  Whereas  the  former  of 
these  is  of  merely  subordinate  importance,  most  of 
the  black  chalks  being  prepared  artificially,  graphite 
is  all  the  more  so  because  it  is  employed,  not  only  as 
the  sole  material  for  lead  pencils,  but  also  for  making 
graphite  crucibles,  as  blacklead  stove  polish,  as  a 
lubricant,  etc.  One  of  its  numerous  applications  is 
in  connection  with  the  electro  deposition  of  metals,  its 
high  electrical  conductivity  causing  it  to  be  used  for 
coating  the  interior  of  the  moulds  in  which  this  deposi- 
tion is  effected. 


Graphite,  also  known  as  plumbago  or  blacklead, 
consists  of  carbon.  It  is  usually  spoken  of  as  pure 
carbon,  but  from  a  very  large  number  of  carefully 
conducted  analyses,  it  would  appear  that  native 
graphite  is  never  quite  pure,  even  the  finest  grades  of 
the  mineral  containing  96-8%  of  carbon  at  the  most. 
The  accompanying  substances — which  in  some  cases 



form  nearly  50%  of  the  whole — are  of  divergent  com- 
position and  consist  of  iron,  silica,  lime,  magnesia  and 
alkalis.  Even  the  combustible  constituent  of  graphite 
is  not  pure  carbon,  but  always  contains  a  certain— 
though  small — proportion  of  volatile  substances.  These 
slight  traces  of  volatile  matter  are  of  considerable 
importance  in  connection  with  the  hypothesis  on  the 
origin  of  the  mineral. 

Contrary  to  the  old  idea,  it  is  now  almost  universally 
considered  that,  instead  of  being  of  volcanic  origin, 
graphite  consists  of  the  remains  of  long-dead  organisms, 
and  in  this  respect  is  closely  related  to  coal.  This 
hypothesis,  however,  fails  to  explain  one  point,  namely 
the  crystalline  nature  of  graphite ;  for  even  anthracites, 
which  form  the  oldest  coals  known  to  have  had  their 
origin  in  the  decomposition  of  organic  substances,  do 
not  reveal  the  faintest  traces  of  crystalline  structure. 
The  upholders  of  the  theory  that  graphite  was  formed 
by  the  action  of  plutonic  forces  adduce,  in  support, 
the  fact  that  graphite  can  actually  be  produced,  in 
certain  chemical  processes,  at  high  temperature. 
Molten  cast-iron  in  cooling  causes  the  separation  of 
carbon  in  the  form  of  graphite ;  and  the  same  substance 
is  also  formed,  in  large  quantities,  in  gas  retorts, 
through  the  decomposition  of  certain  carbonaceous 
compounds  when  brought  into  contact  with  the 
glowing  walls  of  the  retorts.  Recent  investigations, 
however,  have  shown  that  the  temperature  necessary 
for  the  transformation  of  non-crystalline  carbon  into' 
crystalline  graphite  is  by  no  means  so  high  as  was 
formerly  supposed ;  and  it  is  now  known  that  the  change 
takes  place  at  as  low  as  red  heat.  Possibly  the  two 
theories  could  be  reconciled  by  the  assumption  of  a 
very  old  coal — such  as  is  found,  for  instance,  as  anthra- 



cite  in  many  parts  of  the  world — being  so  strongly 
heated,  by  plutonic  action,  as  to  change  into 

Native  graphite  crystallises  in  the  form  of  hexagons, 
mostly  tabular ;  but  really  well -developed  crystals  are 
of  extremely  rare  occurrence,  and  by  far  the  greatest 
quantities  of  this  mineral  are  found  in  the  condition 
of  dense  lumps,  in  which  only  the  crystalline  structure, 
and  not  any  decided  crystals,  can  be  discerned.  The 
hardness  of  the  mineral  fluctuates  within  fairly  wide 
limits,  ranging  from  0-5  to  ro.  The  sp.  gr.  averages 
r8oi8-r844,  but,  in  the  case  of  impure  lumps  may 
increase  to  1-9-2-2. 

The  following  analyses  will  give  some  idea  of  the 
considerable  divergence  existing  between  graphites 
from  different  deposits  :— 

Ash  . 







Carbon          . 

Water  (chemically  combined) 










Lime  . 

Ferric  oxide 


Water  and  volatiles 














Carbon      .... 




Ash           .... 







Carbon      .... 




Silica         .... 




Alumina   .... 




Ferric  oxide 


Manganese  protoperoxide   . 
Lime         .... 








Sulphur    .... 




Loss  on  incineration  . 

.  — 



Of  these  Styrian  specimens,  Nos.  1-4  are  crude 
kinds,  of  sp.  gr.  2*1443;  No.  5  was  levigated  in  the 
laboratory,  and  No.  6  was  levigated  from  an  inferior 
quality  at  the  mine. 

According  to  the  character  of  the  crystalline 
structure,  the  colour  of  graphite  varies,  but  is  mostly 
deep  black.  Very  pure  specimens,  such  as  the  beau- 
tiful graphite  blocks  (from  the  renowned  Alibert 
graphite  mines  in  Siberia)  which,  as  a  rule,  are  only 
to  be  seen  in  exhibitions  and  mineralogical  collections, 
have  the  appearance  of  unpolished  steel  or  white  pig 
iron  (spiegeleisen) .  The  most  important  property  of 
native  graphite  is  its  low  hardness  and  cohesion,  in 
consequence  of  which  it  leaves  a  streak  when  drawn 
over  the  surface  of  paper. 

Graphite  seems  to  be  of  frequent  occurrence  all  over 
the  world,  though  only  few  deposits  are  known  which 
yield  a  product  that  is  suitable  for  all  the  purposes  to 
which  graphite  is  applied. 

In  European  countries,  Austria  is  particularly  rich 
in  graphite;  and  very  large  deposits  of  this  mineral 
are  found  in  Bohemia.  Considerable  deposits  also 
occur  in  Bavaria,  where  they  have  long  been  worked. 


English  graphite  is  celebrated  for  its  excellent  quality. 
All  these  European  deposits,  however,  are  surpassed, 
both  in  extent  and  in  the  quality  of  their  products,  by 
those  discovered  in  Siberia,  the  largest  being  that 
producing  the  aforesaid  Alibert  graphite  and  situated, 
near  the  Chinese  frontier,  in  eastern  Siberia.  At  one 
time,  America  imported  all  her  blacklead  pencils  from 
Europe,  having,  at  that  period,  no  known  graphite 
deposits  furnishing  a  suitable  product.  At  present, 
however,  deposits  of  this  kind  have  been  found  in 
California,  and  there  can  be  little  doubt  but  that  many 
others  of  this  valuable  mineral  remain  to  be  discovered 
in  that  enormous  continent,  the  geological  investigation 
of  which  is  still  far  from  being  complete. 

The  graphite  of  some  deposits  is  so  highly  con- 
taminated by  extraneous  minerals  that  it  cannot  be 
utilised,  since  the  cost  of  purification  would  exceed  the 
value  of  the  product.  On  the  other  hand,  the  purer 
kinds,  when  suitably  refined,  yield  a  graphite  that  is 
fully  adapted  to  all  requirements. 

The  refining  process  may  be  either  chemical  or 
mechanical,  the  choice  of  methods  depending  entirely 
on  the  character  of  the  associated  minerals.  If  these 
mainly  consist  of  coarse,  stony  fragments,  preference 
should  be  given  to  mechanical  treatment ;  but  if  they 
are  of  such  a  character  that  they  cannot  be  eliminated 
in  this  way,  chemical  methods  must  be  employed. 
Sometimes  the  two  systems  are  combined,  by  first 
subjecting  the  graphite  to  a  rough  mechanical  purifi- 
cation, and  then  completing  the  operation  with  chemical 

The  mechanical  treatment  consists  in  first  removing 
as  many  of  the  impurities  as  possible  by  hand-picking, 
and  grinding  the  remainder  in  edge -runner  mills,  along 


with  water.  The  turbid  liquid,  containing  the  powdered 
graphite  and  extraneous  minerals  in  suspension,  is 
led  through  long  launders,  the  sides  of  which  are 
notched  at  intervals  to  allow  the  water  to  overflow  into 
large  pits.  The  graphite  settling  in  the  first  of  these 
pits  contains  numerous  particles  of  the  heavy  associ- 
ated minerals;  but  that  remaining  suspended  in  the 
water  and  carried  on  to  the  further  pits  constitutes 
the  bulk.  The  water  is  left  to  clarify  completely  in 
the  pits,  and  is  then  drawn  off,  the  pasty  residue  being 
shaped  into  prisms,  which  are  compressed  under  heavy 
pressure,  to  increase  their  density,  when  partially  d^. 

Although  levigation  will  remove  most  of  the  accom- 
panying extraneous  minerals,  it  cannot  eliminate  the 
ash  constituents  of  the  graphite.  Experiments  made 
in  this  direction  have  demonstrated  that  the  ash 
content  of  the  levigated  graphite  is  exactly  the  same 
as  that  of  the  crude  material.  Whilst  these  ash  con- 
stituents do  not  affect  the  quality  of  graphite  for  cer- 
tain of  its  uses,  they  nevertheless  impair  its  beautiful 
black  colour  to  a  considerable  degree.  The  chemical 
treatment  necessary  to  eliminate  these  constituents  is 
attended  with  many  difficulties,  the  chief  of  which 
resides  in  the  fact  that  the  ferric  oxide  present  is  in  a 
form  that  is  not  readily  accessible  to  the  action  of 
chemicals.  For  this  reason,  attempts  to  purify  graphite 
with  crude  hydrochloric  acid  are  hardly  likely  to  prove 
successful,  since  both  the  ferric  oxide  and  the  accom- 
panying silicates  obstinately  resist  the  action  of  this 

In  order  to  obtain  graphite  of  a  high  state  of  purity, 
the  attempt  must  be  made  to  bring  this  ferric  oxide 
and  the  silicates  into  a  soluble  condition.  This  can 
be  accomplished  in  various  ways,  and  the  choice  of 


the  method  will  depend  on  the  purpose  for  which  the 
graphite  is  intended.  For  example,  the  operations  may 
either  be  confined  to  purification,  or  else  include  the 
attainment  of  a  maximum  condition  of  subdivision. 
When  foliaceous  graphite  has  to  be  treated — and  this 
kind  of  graphite  cannot,  in  its  original  condition,  be 
used  for  making  lead  pencils — it  is  preferable  to  employ 
a  method  which  will  produce  both  the  above  results. 
The  purification  may  consist  in  crushing  the  graphite 
to  powder,  and  fusing  this  with  a  mixture  of  sulphur 
and  carbonate  of  soda,  whereby  the  silicates  present 
are  converted  into  soluble  compounds,  and  the  ferric 
oxide  into  ferric  sulphide.  On  extracting  the  melt 
with  water,  a  portion  of  the  contained  salts  pass 
into  solution  and  is  carried  off.  The  residue  is  then 
treated  with  dilute  hydrochloric  acid,  which  dissolves 
out  the  ferric  sulphide,  with  liberation  of  sulphuretted 
hydrogen,  and  leaves  the  graphite  in  a  very  pure 
condition  after  wrashing. 

In  order  to  render  foliaceous  graphite  suitable  for 
lead  pencils,  a  different  method  is  pursued,  but  should 
only  be  employed  in  special  circumstances,  on  account 
of  the  expense  entailed. 

According  to  the  process  recommended  by  Brodie, 
the  graphite,  ground  to  coarse  powder,  is  mixed  with 
about  one-fourteenth  of  its  own  weight  of  chlorate  of 
potash,  this  mixture  being  heated,  with  two  parts  by 
weight  of  sulphuric  to  each  part  of  graphite,  in  a  water 
bath  so  long  as  fumes  of  hypo  chlorous  acid  continue 
to  be  disengaged.  The  heating  must  be  performed  in 
stoneware  or  porcelain  vessels,  those  made  of  any 
other  materials  being  strongly  corroded  by  the  chlorine 
compounds  formed. 

When  the  evolution  of  fumes  ceases,  the  mass  is 


allowed  to  cool,  and  is  carefully  washed  with  a  large 
volume  of  water,  the  residue  being  then  dried  and  heated 
to  redness.  During  this  calcination  the  graphite  under- 
goes a  peculiar  change,  increasing  considerably  in  bulk 
and  forming  an  exceedingly  soft  powder  which,  after 
another  washing,  consists  almost  entirely  of  chemically 
pure  carbon. 

Graphite  purified  in  this  way  can  be  used  for  any 
purpose  for  which  this  material  is  employed,  and  may 
be  made  into  the  finest  lead  pencils.  However,  as 
already  mentioned,  this  process  is  usually  too  expensive 
for  general  application. 

The  use  of  graphite  for  writing  is  more  ancient  than 
is  usually  supposed,  having  been  tentatively  employed 
between  1540  and  1560.  It  was  during  this  period 
that  the  graphite  mines  in  Cumberland  were  discovered  ; 
and  the  extremely  pure  graphite  found  there  soon  began 
to  be  used  as  a  writing  material. 

Up  to  the  close  of  the  eighteenth  century,  lead  pencils 
were  made  by  selecting  pure  lumps  of  graphite  and 
sawing  them  into  thin  rods,  which  were  then  encased 
in  wooden  sticks.  Apart  from  their  high  price,  these 
pencils  exhibited  various  defects,  one  of  the  chief  being 
that  a  stick  of  such  pencil  was  seldom  of  uniforrh  hard- 
ness throughout  its  length,  most  of  them  being  so  soft 
in  parts  as  to  make  a  deep  black,  smeary  mark,  whilst 
other  parts  would  hardly  give  any  mark  at  all. 

The  defects  inherent  in  native  graphite  are  com- 
pletely removed  by  the  method  now  generally  employed 
in  making  lead  pencils;  and  on  this  account  the  old 
process  of  sawing  the  lumps  has  been  abandoned. 

Graphite  with  a  fine  earthy  texture  alone  is  suitable 
for  lead  pencils,  scaly  varieties  being  useless  for  this 
purpose,  unless  specially  prepared,  since  they  will  not 


give  a  solid  black  streak.  By  means  of  the  Brodie 
process,  however,  even  the  most  highly  crystalline 
kinds  can  be  rendered  suitable  for  this  purpose. 
Siberian  graphite  is  distinguished  by  extremely  high 
covering  power,  and  is  specially  preferred  for  the 
manufacture  of  pencils.  Excellent  varieties  for  this 
purpose  are  also  found  in  many  parts  of  Europe ;  and 
indeed,  a  large  proportion  of  all  the  lead  pencils  used 
throughout  the  world  are  made  from  Bohemian,  Styrian 
and  Bavarian  graphite. 

At  present,  all  pencils  are  made  from  ground  graphite, 
the  extremely  finely  ground  and  levigated  material 
being  kneaded  into  a  paste  with  clay.  This  operation 
fulfils  a  twofold  purpose,  the  plasticity  of  the  clay 
increasing  the  cohesion  of  the  individual  particles  of 
graphite,  whilst  the  amount  of  clay  used  determines 
the  hardness  of  the  pencil. 

The  larger  the  proportion  of  clay,  the  harder  the 
pencil  when  baked,  and  therefore  the  paler  the  mark 
the  pencil  will  make  on  paper.  In  the  pencil  factories, 
the  clay  is  incorporated  in  special  machines;  and  the 
operation  requires  extreme  care,  since  only  a  perfectly 
uniform  mixture  will  give  a  composition  of  regular 
character  in  all  cases. 

The  intimately  mixed  material  is  formed  into  thin 
rods,  which  are  dried  and  then  baked,  the  heat  driving 
out  the  water  in  the  clay  and  transforming  it  into  a 
solid  mass. 

An  addition  to  this  main  application  of  graphite, 
the  mineral  is  also  used  for  making  crucibles,  chiefly 
for  melting  the  noble  metals.  Crucibles  of  this  kind 
are  largely  manufactured  near  Passau,  Bavaria,  and 
similar  crucibles  are  made  in  England  from  Ceylon 


Another  important  use  for  graphite  is  as  a  coating 
for  iron  articles  to  protect  them  from  rust.  For  this 
purpose,  however,  only  the  inferior  kinds  are  employed ; 
and  these  can  also  be  made  up  into  excellent  cements 
capable,  in  particular,  of  offering  considerable  resistance 
to  the  action  of  heat  and  chemicals. 

To  complete  the  tale  of  the  applications  of  graphite, 
its  employment  as  a  lubricating  agent  for  machinery, 
especially  for  reducing  friction  in  machines  made  of 
wood,  may  be  mentioned.  Latterly  also,  the  finest 
levigated  graphite  has  come  into  use,  in  admixture 
with  solid  fats  or  mineral  oils,  for  lubricating  large 
engines,  for  which  purpose  it  is  excellently  adapted. 


Black  chalk,  slate  black,  Spanish  chalk,  crayon,  etc., 
is  not  a  chalk  at  all,  in  the  mineralogical  sense,  but 
consists  of  clay  shale  of  varying  colour.  Some  kinds 
of  this  shale  are  pure  black,  almost  velvet  black,  and 
these  are  considered  the  best.  Others  have  a  more 
greyish  or  bluish  tinge  and  are  of  low  value  as 

The  purer  the  black,  the  finer  the  grain  of  the 
material,  and  therefore  the  greater  its  value  to  the 
colour-maker.  The  variety  obtained  from  Spain  is 
generally  admitted  to  be  the  best,  and  for  this  reason 
the  name  of  Spanish  chalk  has  been  applied  to  all 
similar  minerals. 

In  all  cases  the  black  colour  of  Spanish  chalk  is  due 
to  carbon;  but  the  particular  modification  of  carbon 
present  has  not  yet  been  accurately  identified.  Accord- 
ing to  some,  it  is  chiefly  graphite,  whereas  others  ascribe 
the  colour  to  amorphous  carbon.  Apparently,  the 


material  found  in  different  deposits  contains  either  one 
or  the  other  of  these  modifications  of  carbon. 

Deposits  of  black  chalk  are  fairly  plentiful,  but  in 
many  of  them  the  material  is  so  contaminated  with 
extraneous  minerals  that  a  somewhat  troublesome 
method  of  preparation  is  needed  to  fit  them  for  the 
purpose  of  the  draughtsman.  With  this  object,  the 
native  product  must  be  ground  extremely  fine,  and 
the  powder  levigated;  and  owing  to  the  expense  of 
these  processes,  they  are  now  seldom  used,  it  being 
possible  to  obtain  a  good  black  chalk  far  more  cheaply 
than  by  levigating  the  natural  material. 

This  artificial  black  chalk  is  prepared  by  mixing 
ordinary  white  chalk,  or  white  clay,  with  a  black 
colouring  matter,  shaping  the  mass  into  prisms,  and 
sawing  these  into  suitable  pieces  when  dry.  The 
white  pigment  may  either  be  mixed  with  some  very 
deep  black  substance,  such  as  lampblack,  or  stained 
with  an  organic  dyestuff ,  which  is,  in  reality,  not  black, 
but  either  very  dark  blue  or  green. 

The  usual  colouring  matter  used  with  white  chalk 
is  lampblack,  mixed  to  a  uniform  paste  with  thin  glue, 
a  suitable  amount  of  clay  or  chalk  being  incorporated 
with  the  mass.  The  production  of  a  perfectly  homo- 
geneous mixture  entails  subjecting  the  paste  to  a 
somewhat  protracted  mechanical  treatment.  When 
the  mass  has  become  perfectly  uniform  throughout, 
it  is  shaped  into  prisms,  which  are  exposed  to  the  air 
to  dry  and  are  then  cut  up  with  a  saw.  Instead  of 
prisms,  the  mass  can  be  shaped  into  thin  sticks,  which 
dry  more  quickly. 

A  very  handsome  black  chalk  can  be  made,  with 
comparatively  little  trouble,  by  treating  chalk  with  a 
suitable  quantity  of  logwood  decoction  previously 


mixed  with  sufficient  green  vitriol  solution  to  render 
the  liquid  a  deep  black.  This  liquid  is  added  to  the 
dry  chalk,  intimately  mixed  therewith,  and  the  pasty 
mass  shaped  into  sticks.  The  colouring  agent  may  be 
replaced  by  a  solution  of  logwood  extract  blackened 
by  the  addition  of  a  small  quantity  of  chromate  of 
potash ;  or  black  dyestuffs  may  be  used. 



MENTION  has  already  been  made  of  the  great  con- 
fusion prevailing  in  the  nomenclature  of  pigments, 
and  that  many  of  these  are  put  on  the  market  under 
a  variety  of  names  taken  from  different  languages. 

Although  the  number  of  the  earth  colours  is  far 
smaller  than  that  of  the  artificial  colouring  matters, 
the  nomenclature  is  in  a  no  less  confused  condition. 

Most  frequently,  earth  colours  are  named  after  the 
localities  where  they  are  either  discovered  or  prepared, 
in  combination  with  the  word  indicating  the  colour 
of  the  product — for  example :  Cologne  white,  Vienna 
white — or  the  term  "  earth  "  (Verona  earth,  Veronese 
green,  etc.).  Whilst  these  names  give,  to  some  extent, 
an  indication  of  the  nature  of  the  pigment,  others 
have  no  reference  to  it  at  all;  such  as  colcothar,  bole, 
umber,  etc.  Finally,  a  number  of  other  names  in 
use  are  calculated  to  produce  the  impression  that  the 
earth  colours  in  question  are  of  an  entirely  different 
nature  to  their  real  one.  As  an  example,  we  may 
cite  the  name  "  French  chalk,"  which  is  not  a  chalk 
at  all,  but  consists  of  the  mineral  talc.  Black  chalk, 
again,  is  not  chalk  (calcium  carbonate),  but  a  black 
shale;  and  graphite  is  often  termed  blacklead, 
although  it  contains  no  lead  at  all,  and  the  name  is 



merely  a  survival  from  the  time  when  pencils  of 
metallic  lead  were  used  for  drawing. 

In  order  to  bring  some  kind  of  order  into  the  various 
names  which  are  applied  to  the  earth  colours,  a  list 
of  those  in  current  use  is  appended.  Many  of  these 
names,  it  may  be  stated,  have  been  selected  in  a  purely 
arbitrary  manner,  some  manufacturers,  for  instance, 
selling  ordinary  chalk  under  a  variety  of  foreign 
names,  for  the  purpose  of  thereby  obtaining  higher 
prices.  These  borrowed  names  would  seem  to  be 
superfluous,  to  say  the  least.  Pure  and  properly 
levigated  chalk  is  the  same  article  everywhere,  whether 
prepared  from  English,  French  or  German  limestone; 
and  in  all  cases  the  simple  name,  "  chalk,"  with  an 
explanatory  "  single,"  "  double,"  or  "  triple  "  levi- 
gated, should  be  quite  sufficient. 

In  the  case  of  earth  colours  that  are  really  obtained 
of  special  quality  in  certain  localities,  such  as  terra  di 
Siena,  green  earth  from  Verona,  or  the  like,  the  corre- 
sponding name  might  be  retained,  even  if  the  pigment 
did  not  originate  from  the  locality  in  question,  as  a 
generic  term  for  a  pigment  possessing  certain  properties 
and  of  a  certain  composition. 

In  the  following  classification,  the  names  of  the  earth 
colours  are  given  in  accordance  with  their  colour  and 
chemical  composition. 


Carbonate  of  Lime : 

Chalk;  levigated  chalk;  Vienna  white;  Spanish 
white ;  marble  white ;  artists'  white ;  Bougival  white ; 
Champagne  chalk;  Paris  chalk;  Cologne  chalk; 
Mountain  chalk;  craie;  blanc  mineral;  Blanc  de 


Champagne;  Blanc  de  Meudon;  Blanc  de  Bougival; 
Blanc  de  Troyes;  Blanc  d' Orleans;  Blanc  de  Rouen; 
Blanc  de  Briancon. 

Basic  Carbonate  of  Lime  : 

Vienna  white ;  Vienna  lime ;  pearl  white ;  whiting ; 
Blanc  de  chaux ;  Blanc  de  Vienne. 

Note. — The  calcareous  marls,  consisting  of  carbonate 
of  lime  and  clay,  are  also  frequently  sold  under  the 
above  names,  the  same  being  the  case  with  gypsum. 

Silicate  of  Alumina  : 

White  earth;  pipeclay;  Dutch  white;  Cologne 
earth ;  terre  d' Argile ;  Argile  blanc ;  Terre  blanche. 

Silicate  of  Magnesia  (mineralogically,  talc  and 
soapstone)  : 

Talc;  Venetian  earth;  French  chalk;  Venetian 
white;  glossy  white;  feather  white;  shale  white; 
face-powder  white ;  Blanc  de  Venise,  Blanc  d'Espagne ; 
Blanc  de  fard. 

Note. — Fine  grades  of  white  lead  are  also  sold  as 
Venetian  white,  Spanish  white  and  shale  white;  but 
can  easily  be  recognised  by  their  weight.  The  term 
"prepared"  white,  frequently  applied  to  earth 
colours  in  the  trade,  usually  indicates  that  the  material 
in  question  has  been  either  levigated,  ground  or  burnt — 
in  short,  put  through  some  kind  of  preparatory  treat- 
ment— and  is  therefore  in  frequent  use  for  all  the 

Barium   sulphate : 

Heavy  spar;  barytes;  heavy  earth;   mineral  white. 

Precipitated  colours  : 
Permanent  white ;  blanc  fixe. 



Ferric  hydroxide,  with  admixtures  of  ferric  oxide, 
clay,  lime,  ferric  silicate,  basic  ferric  sulphate,  etc. 

Ochre;  iron  ochre;  golden  ochre;  satin  ochre 
(satinober) ;  pit  ochre ;  vitriol  ochre ;  Mars  yellow ; 
Chinese  yellow;  Imperial  yellow;  permanent  yellow; 
terra  di  Siena ;  umber ;  Italian  earth ;  Roman  earth  ; 
bronze  ochre;  oxide  yellow,  etc. 

Yellow  ochre;   JaunedeMars;  Terre  d' Italic. 

Ferric  Silicate : 

Yellow  earth;   Argile  jaune;  yellow  wash. 


Ferric  oxide  (with  alumina  and  silica). 
Bolus ;    bole ;    Terra  sigillata ;    Lemnos  earth ;    red 
chalk;  raddle;   Striegau  earth. 

Ferric  oxide  : 

Colcothar;  English  red;  angel  red;  Pompeii  red; 
Persian  red;  Indian  red;  Berlin  red;  Naples  red; 
Nuremberg  red;  crocus;  chemical  red;  Crocus 
Mart  is  iron  saffron ;  caput  mortuum ;  raddle ;  rouge 
de  fer;  Rouge  de  Perse;  Rouge  des  Indes;  Rouge 
de  Mars;  Rouge  d'Angleterre. 


Ferric  oxide : 

Ferric    hydroxide;     Ferric    silicate    (conf.    Yellow 
Earth  Colours,  which  are  often  sold  under  the  same 


names  as  the  browns.  The  paler  kinds  are  usually 
called  "  pale  "  or  "  golden,"  such  as  pale  ochre,  golden 
ochre,  etc.).  Terra  di  Siena;  burnt  Siena;  satinober; 
mahogany  brown ;  Vandyke  brown. 

Ferric  silicate,  Clay  : 

Umber;  umber  brown;  Roman  earth;  Roman 
umber;  Turkish  brown;  Sicilian  brown;  Cyprus 
earth ;  chestnut  brown ;  burnt  umber ;  ombre ;  Terre 
d'ombre ;  Ombre  brulee. 

Organic  decomposition  products  : 

Cologne  umber;  Cologne  earth;  Cassel  brown; 
Spanish  brown;  mahogany  brown;  Vandyke  brown; 
brown  carmine;  Terre  brun  de  Cologne;  Brun  de 
Cologne ;  Brun  d'Espagne ;  Ombre  de  Cologne ;  Brun 
de  Cassel;  Terre  d'Ombre;  Cologne  brown. 

Asphaltum  (mineral  rosin)  : 

Asphaltum  brown ;  bistre ;  earth  brown ;  bitumen ; 
pitch  brown ;  Asphalte ;  Brun  de  bitume ;  Bitume. 


Ferrous  oxide  with  silica,  alumina,  lime,  etc.  : 
Green  earth ;  Verona  green ;   Celadon  green ;  Verona 
earth ;    Italian  green ;    stone  green ;    Bohemian  earth ; 
Cyprus  earth;   Tyrol  green;    permanent  green;   green 
ochre;  Terre  verte;  Terre  de  Verone ;  Vert  d'ltalie. 

Cupric  carbonate  : 

Malachite  green;  mountain  green;  Hungarian 
green ;  copper  green ;  mineral  green ;  Tyrolese  green  ; 
shale  green;  Vert  de  montagne;  Vert  d'Hongrie. 



Cupric  carbonate : 

Malachite  blue;  mountain  blue;  lazulite  blue; 
azure  blue;  mineral  blue;  copper  blue;  Hamburg 
blue;  English  blue;  Cendres  bleues;  Bleu  d'azure; 
Bleu  de  cuivre ;  Vert-de-gris  bleu ;  blue  verditer. 


Grey  clay  shale : 

Mineral  grey ;  silver  grey ;  stone  grey ;  slate  grey. 

Carbon : 
Graphite ;  blacklead ;  plumbago ;  iron  black. 

Clay  shale : 

Black  chalk;  slate  black;  Spanish  black;  Spanish 
chalk;  oil  black;  Schiste  noir;  Noir  d'Espagne. 


Alabaster.     See  Gypsum. 
Alumina,  silicate  of,  21,  22 
Aluminium-potassium  silicate, 

Alum  sludge,  32 

— ,  artificial  ochre  from,  148 

— ,  ferric  oxide  pigments  from, 

Ammonium  salts,  artificial  ochre 

from,  145,  146 
Anhydrite,  19 
Anthracolite,  13 
Arragonitc,  13 
Asphaltum,    37,    38.     See.    also 


— ,  brown,  174,  175 
Augite,  179 
Azurite,  33 

Ball  Mills,  55-56 
Barium  carbonate,  20 

—  sulphate.     See  Barytes. 
Barytes,  19,  20,  119-122 

— ,  artificial,  133 
— ,  correcting  colour  of,  121 
— ,  detecting,  in  white  lead, 

— ,  low  covering  power  of,  121 
Black  chalk,  38 
—  earth  colours,  185-196 

— ,  trade  names  of,  202 

—  earths,  6,  38-39 

—  schist,  38 
Bitumen,  174,  175 
Blanc  fixe,  19 

Blue  earth  colours,  183-184 
,  trade  names  of,  202 

—  earths,  4,  33-36 
Bole,  31,  32,  152-154 
Bone  breccia,  13 

Brown  coal,  pigments  from,  175 
—  earth  colours,  168-175 

— ,  trade  names  of,  200 
—  earths,  5,  36—38 

Calcareous  marl,  no,  in 

—  tuff,  12 
Calcining,  81 

—  Ferric  oxide,  161-164 
—  furnaces.     See  Furnaces. 

—  lime,  88-90 

—  ochre,  132-136 
Calcite,  n,  12,  14,  15 
Calcium  carbonate,  12,  14,  15,  16 

— ,  action  of  acids  on,  15 

—  hydroxide,  16 

—  sulphate.     See  Gypsum. 
Calc  sinter,  12 

—  spar.     See  Calcite. 
Caledonian  brown,  36 
Cappagh  brown,  36 

Caput  mortuum.    See  Colcothar. 
Carbon  brown,  37 

in  limestone,  16 

Cassel  brown,  37,  38,  174 
Celadon  green.     See  Green  earth. 
Chalk,  13 
,  black,  194-196 

— ,  colour  of,  103 
,  correcting  colour  of,  104, 

105,  106 

— ,  covering  power  of,  106 

— ,  grinding,  101 

— ,  impurities  in,  103,  104 

— ,  precipitated,  107-103 

— ,  preparation  and  properties 

of,  98-106 
Classification   of   earth   colours, 

Clay,  21-23 




Clay,  formation  of,  113 
— ,  impurities  in,  114-119 

—  in  ochre,  128 

— ,  levigating,  114-117 
Colcothar,  160,  161,  162 
Cologne  earth,  173,  174 
Commercial  nomenclature  of 

earth  colours,  197-202 
Crushers  and  Breakers,  43-45 
Crushing,  77-80 

—  machinery,  43-60 

Disintegrators,  58-60 
Distemper,  weatherproof,  94 
Dolomite,  18 

Draining  and  Drying,  66-77 
Drying  appliances,  73-77 
Dyestuffs   for  improving    earth 
colours,  85 

Edge  runners,  48-55 
English  red,  160 

Ferric  hydroxide  in  ochre,  128- 

oxide,  artificial  ochre  from, 


as  by-product,  30 
,  burnt,  158-164 

— ,  calcining,  161-164 

—  in  lime,  detection  of, 

— ,  native,    as    pigment, 

-  pigments  from  alum 
sludge,  164-167 

— ,  range  of  colours,  29 

—  shading,  28 
,  violet  shades  from, 

Ferrous  sulphate,  artificial  ochre 

from,  139-143,  146-148 
Filter- cloths,  cleaning,  72 
Filter-presses,  70-73 
Furnaces,    calcining,    158,    162, 

163,  166 

Granulator,  43 
Graphite,  38,  39,  185-194 

as  a  lubricant,  194 

—  as  anti-corrosive,  194 

Graphite  in  the  manufacture  of 
lead  pencils,  191-193 

—  for  crucibles,  193 
— ,  refining,  189-192 

Green  earth,  176-180 

,  artificial,  180 

— ,  improving,  178 
—  colours,  176-184 
— ,  trade  names  of,  201 

—  earths,  5 

Grey  earth  colours,  trade  names 

of,  202 

Grey  earths,  38 
Gypsum,  18/19,  in,  112 

Heavy  spar.     See  Barytes. 
Hematite,  155 

— ,  brown,  23,  30,  31 

— ,  red,  28,  30 
Hydro -extractor,  66-70 

Improving  earth  colours,  84,  85 
Indian  red,  29,  160 
Iron  cream,  29 

—  glance,  154 

in  limestone,  17 

ore,  bog,  25,  31 

— ,  micaceous,  28 
Ironstone,  brown,  23,  24,  25 

— ,  clay,  24 
,  red,  28-30 

Kaolin,  21,  22,  112-119 

Lazulite,  183 

Lemnos  earth.     See  Bole. 

Levigation,  60-65 

Lime,     absorption     of     carbon 

dioxide  by,  93 

— ,  action  of,  on  casein,  94 

, ,  on  colours,  93,  98 

,  calcining,  88-90 

— . — ,    caustic,    preparation    of, 

,  double  compound  of  oxide 

and  carbonate,  93 

—  from  mussel  shells,  98 
— ,  impurities  in,  91,  92 

—  in  clay,  22 

,     eliminating,      117- 

in  ochre,  129 



Lime  in  the  preparation  of  arti- 
ficial ochre,  140-144 
— ,  moulding,  96-98 

,  quick,  16 

— ,  slaked,  16 

Limestone,  11-18 

,  suitability  of,  for  colour- 
making,  92 

Limonite,  25 

Magnesia,  carbonate  of,  123,  124 

—  in  lime,  91 

—  in  limestone,  17 
Magnesium  silicate,  21 
Malachite,  35 

-blue,  183 
Marble,  n,  14,  15 
Minerals,  testing  for  suitability 

as  pigments,  172 
Mine  sludge,  32 
Mixing  earth  colours,  81-84 
Moulding,  85,  86 
Mountain  chalk,  12 

milk,  12 

Muffle,    burning   ochre  in    the, 

Muriacite,  19 
Muschelkalk,  13 

Ochre,  24,  25,  26 

,  blue.     See  Vivianite. 

,  calcining,  132-136 

— ,  English,  138 
,  green,  180 

— ,  pit,  148-150 

— ,  Roman,  137,  138 

— ,  Siena,  137,  138 

—.testing,  130-132 

— ,  toning  with  chalk,  144 

— ,  toning  with  clay,  144 

— ,  vitriol,  146-148 
Ochres,  128-150 

— ,  artificial,  138-146 

—  as  by-products,  146-150 
,  burnt,  158-164 

—  from  various  deposits,  136- 


,  Italian,  137,  138 

Oolithic  limestone,  13 
Organic  matter  in  lime,  91 

Pastel  crayons,  126 

Pearl  white,  94 
Permanent  white,  19,  122 
Pipeclay.     See  Kaolin. 
Preparation  of  colour  earths,  40- 

Pulverisers,  56-58 

Raddle,  29,  155-158 
,  impurities  in,  156 

.testing,  157 

Raw  materials  for  earth  colours, 

Red  earth  colours,  151-167 

,  trade  names  of,  200 

Red  earths,  4,  27-33 

Sampling  raw  earths,  9 

Selenite,  18 

Shading  pigments  with  perma- 
nent white,  19 

Siena,  Terra  di,  25,  26,  27,  168- 

, .  See  also  Italian 


Siderosilicate,  171 

Sifting,  77-80 

Soapstone,  20,  21.  See  also 

Spanish  brown,  174 

Sprudelstein,  15 

Steatite,  20,  21,  125,  126 

Stamps,  45-48 

Talc,  20,21,  124,  125 
Terra  sigillata.     See  Bole. 
Testing    purity    of  raw   earths, 

Trade  names  of  earth  colours, 


Ultramarine,  33 
Umber,  36,  170-174 

,  Cologne,  173,  174 

,  true,  170-173 

Vandyke  Brown,  38,  174 
Vermilion,  151 

Verona  earth.     See  Green  earth. 
Vienna  white,  95-98 
Vivianite,  33,  34,  184 

White  earth  colours,  87-126 

206  INDEX 

White   earth,   trade  names    of,       Working  earth  colour  deposits,  9 

in,  198,  199 

White  earths,  4  Yellow  earth  colours,  127-150 

White  raw  materials  and   pig- ,  trade  names  of,  100 

mentary  earths,  11-23  Yellow  earth,  150 

Witherite,  20  Yellow  earths,  4,  23-27 


The  Manufacture  of  Paint 




Second  Edition— Revised  and  Enlarged 

DEMY  8vo.     73  ILLUSTRATIONS.    300  PAGES 











XVI. — Cost  Charges — Cost  of  Handling — Carriage  and 
Delivery  of  Goods — Cost  of  Materials — Machinery 
as  affecting  Manufacturing  Cost — Electricity  as 
Motive  Power — Manufacturing  Oncost — Prices — 
The  Future  of  the  Industry. 

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The  Chemistry  of  Pigments 

BY  ERNEST  J.  PARRY,  B.Sc.,  (LOND.),  F.I.C.,  F.C.S. 


JOHN   H.  COSTE,  F.I.C.,  F.C.S. 

Demy   8vo.  5  Illustrations.  280   Pages 

Chapter  I. — Introductory 

Light— White  Light— The  Spectrum—The  Invisible  Spectrum 
—Normal  Spectrum — Simple  Nature  of  Pure  Spectral  Colour 
— The  Recomposition  of  White  Light — Primary  and  Comple- 
mentary Colours — Coloured  Bodies — Absorption  Spectra. 

Chapter  II.— The  Application  of  Pigments 

Uses  of  Pigments :  Artistic,  Decorative,  Protective  — 
Methods  of  Application  of  Pigments  :  Pastels  and  Crayons, 
Water  Colour,  Tempera  Painting,  Fresco,  Encaustic  Painting, 
Oil-Colour  Painting,  Ceramic  Art,  Enamel,  Stained  and 
Painted  Glass,  Mosaic. 

Chapter  III. — Inorganic  Pigments 

White  Lead— Zinc  White— Enamel  White— Whitening— Red 
Lead — Litharge — Vermilion — Royal  Scarlet — The  Chromium 
Greens — Chromates  of  Lead,  Zinc,  Silver  and  Mercury — 
Brunswick  Green — The  Ochres — Indian  Red — Venetian  Red — 
Siennas  and  Umbers — Light  Red — Cappagh  Brown — Red 
Oxides  —  Mars  Colours — Terre  Verte  —  Prussian  Brown  — 
Cobalt  Colours — Coaruleum  —  Smalt  —  Copper  Pigments  — 
Malachite — Bremen  Green — Scheele's  Green — Emerald  Green 
— Verdigris  —  Brunswick  Green  —  Non-arsenical  Greens — 
Copper  Blues — Ultramarine — Carbon  Pigments — Ivory  Black 
— Lamp  Black — Bistre-— Naples  Yellow— Arsenic  Sulphides  : 
Orpiment,  Realgar — Cadmium  Yellow — Vandyck  Brown. 

Chapter  IV.— Organic  Pigments 

Prussian  Blue — Natural  Lakes — Cochineal — Carmine — Crim- 
son— Lac  Dye — Scarlet —  Madder — Alizarin — Campeachy — 
Quercitron — Rhamnus — Brazil  Wood — Alkanet — Santal  Wood 
— Archil — Coal-tar  Lakes — Red  Lakes — Alizarin  Compounds — 
Orange  and  Yellow  Lakes — Green  and  Blue  Lakes — Indigo- 
Dragon's  Blood — Gamboge — Sepia — Indian  Yellow,  Puree 
— Bitumen,  Asphaltum,  Mummy — Index. 

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O  PQ 






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